xfs: use XFS_B_TO_FSB in xfs_ioctl_setattr

Clean up xfs_ioctl_setattr a bit by using XFS_B_TO_FSB.

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