NFS: Fix a page leak in nfs_destroy_unlinked_subrequests()

When we detach a subrequest from the list, we must also release the
reference it holds to the parent.

Fixes: 5b2b5187 ("NFS: Fix nfs_page_group_destroy() and nfs_lock_and_join_requests() race cases")
Cc: stable@vger.kernel.org # v4.14+
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/write.c

@@ -444,6 +444,7 @@ nfs_destroy_unlinked_subrequests(struct nfs_page *destroy_list,
 		}
 
 		subreq->wb_head = subreq;
+		nfs_release_request(old_head);
 
 		if (test_and_clear_bit(PG_INODE_REF, &subreq->wb_flags)) {
 			nfs_release_request(subreq);