xfs: remove the buffered I/O fallback assert

The iomap code has been designed from the start not to do magic fallback,
so remove the assert in preparation for further code cleanups.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_file.c

@@ -609,12 +609,6 @@ xfs_file_dio_write(
 out:
 	if (iolock)
 		xfs_iunlock(ip, iolock);
-
-	/*
-	 * No fallback to buffered IO after short writes for XFS, direct I/O
-	 * will either complete fully or return an error.
-	 */
-	ASSERT(ret < 0 || ret == count);
 	return ret;
 }