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

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

[ Upstream commit ee20808d848c87a51e176706d81b95a21747d6cf ]

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>
Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>