NFS: Fix a compile warning when !(CONFIG_NFS_V3 || CONFIG_NFS_V4)

gcc reports:

linux/fs/nfs/write.c: In function ‘nfs_page_find_head_request_locked.isra.17’:
linux/fs/nfs/write.c:121:64: warning: ‘cinfo.mds’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  list_for_each_entry_safe(freq, t, &cinfo.mds->list, wb_list) {
                                                                  ^
linux/fs/nfs/write.c:110:25: note: ‘cinfo.mds’ was declared here
  struct nfs_commit_info cinfo;
Reported-by: NAnna Schumaker <Anna.Schumaker@netapp.com>
Cc: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/write.c

@@ -49,6 +49,9 @@ static const struct nfs_rw_ops nfs_rw_write_ops;
 static void nfs_clear_request_commit(struct nfs_page *req);
 static void nfs_init_cinfo_from_inode(struct nfs_commit_info *cinfo,
 				      struct inode *inode);
+static struct nfs_page *
+nfs_page_search_commits_for_head_request_locked(struct nfs_inode *nfsi,
+						struct page *page);
 
 static struct kmem_cache *nfs_wdata_cachep;
 static mempool_t *nfs_wdata_mempool;
@@ -94,38 +97,6 @@ static void nfs_context_set_write_error(struct nfs_open_context *ctx, int error)
 	set_bit(NFS_CONTEXT_ERROR_WRITE, &ctx->flags);
 }
 
-/*
- * nfs_page_search_commits_for_head_request_locked
- *
- * Search through commit lists on @inode for the head request for @page.
- * Must be called while holding the inode (which is cinfo) lock.
- *
- * Returns the head request if found, or NULL if not found.
- */
-static struct nfs_page *
-nfs_page_search_commits_for_head_request_locked(struct nfs_inode *nfsi,
-						struct page *page)
-{
-	struct nfs_page *freq, *t;
-	struct nfs_commit_info cinfo;
-	struct inode *inode = &nfsi->vfs_inode;
-
-	nfs_init_cinfo_from_inode(&cinfo, inode);
-
-	/* search through pnfs commit lists */
-	freq = pnfs_search_commit_reqs(inode, &cinfo, page);
-	if (freq)
-		return freq->wb_head;
-
-	/* Linearly search the commit list for the correct request */
-	list_for_each_entry_safe(freq, t, &cinfo.mds->list, wb_list) {
-		if (freq->wb_page == page)
-			return freq->wb_head;
-	}
-
-	return NULL;
-}
-
 /*
  * nfs_page_find_head_request_locked - find head request associated with @page
  *
@@ -750,6 +721,38 @@ nfs_mark_request_dirty(struct nfs_page *req)
 }
 
 #if IS_ENABLED(CONFIG_NFS_V3) || IS_ENABLED(CONFIG_NFS_V4)
+/*
+ * nfs_page_search_commits_for_head_request_locked
+ *
+ * Search through commit lists on @inode for the head request for @page.
+ * Must be called while holding the inode (which is cinfo) lock.
+ *
+ * Returns the head request if found, or NULL if not found.
+ */
+static struct nfs_page *
+nfs_page_search_commits_for_head_request_locked(struct nfs_inode *nfsi,
+						struct page *page)
+{
+	struct nfs_page *freq, *t;
+	struct nfs_commit_info cinfo;
+	struct inode *inode = &nfsi->vfs_inode;
+
+	nfs_init_cinfo_from_inode(&cinfo, inode);
+
+	/* search through pnfs commit lists */
+	freq = pnfs_search_commit_reqs(inode, &cinfo, page);
+	if (freq)
+		return freq->wb_head;
+
+	/* Linearly search the commit list for the correct request */
+	list_for_each_entry_safe(freq, t, &cinfo.mds->list, wb_list) {
+		if (freq->wb_page == page)
+			return freq->wb_head;
+	}
+
+	return NULL;
+}
+
 /**
  * nfs_request_add_commit_list - add request to a commit list
  * @req: pointer to a struct nfs_page
@@ -868,6 +871,13 @@ int nfs_write_need_commit(struct nfs_pgio_header *hdr)
 }
 
 #else
+static struct nfs_page *
+nfs_page_search_commits_for_head_request_locked(struct nfs_inode *nfsi,
+						struct page *page)
+{
+	return NULL;
+}
+
 static void nfs_init_cinfo_from_inode(struct nfs_commit_info *cinfo,
 				      struct inode *inode)
 {