xfs: create a new helper to return a file's allocation unit

Create a new helper function to calculate the fundamental allocation
unit (i.e. the smallest unit of space we can allocate) of a file.
Things are going to get hairy with range-exchange on the realtime
device, so prepare for this now.

Remove the static attribute from xfs_is_falloc_aligned since the next
patch will need it.

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