nfsd: Make __get_nfsdfs_client() static

Fix sparse warning:

fs/nfsd/nfsctl.c:1221:22: warning:
 symbol '__get_nfsdfs_client' was not declared. Should it be static?
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfsctl.c

@@ -1218,7 +1218,7 @@ static void clear_ncl(struct inode *inode)
 }
 
 
-struct nfsdfs_client *__get_nfsdfs_client(struct inode *inode)
+static struct nfsdfs_client *__get_nfsdfs_client(struct inode *inode)
 {
 	struct nfsdfs_client *nc = inode->i_private;