xfs: don't open code XFS_FILBLKS_MIN in xfs_bmapi_write

XFS_FILBLKS_MIN uses min_t and thus does the comparison using the correct
xfs_filblks_t type.  Use it in xfs_bmapi_write and slightly adjust the
comment document th potential pitfall to take account of this

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>