NFS: checking for NULL instead of IS_ERR() in nfs_commit_file()

nfs_create_request() doesn't return NULL, it returns error pointers.

Fixes: 67911c8f ('NFS: Add nfs_commit_file()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>