xfs: fix fallocate functions when rtextsize is larger than 1rt-fixes-5.10_2020-10-10

In commit fe341eb151ec, I forgot that xfs_free_file_space isn't strictly
a "remove mapped blocks" function.  It is actually a function to zero
file space by punching out the middle and writing zeroes to the
unaligned ends of the specified range.  Therefore, putting a rtextsize
alignment check in that function is wrong because that breaks unaligned
ZERO_RANGE on the realtime volume.

Furthermore, xfs_file_fallocate already has alignment checks for the
functions require the file range to be aligned to the size of a
fundamental allocation unit (which is 1 FSB on the data volume and 1 rt
extent on the realtime volume).  Create a new helper to return the
desired allocation unit size, fix the fallocate frontend to use it,
fix free_file_space to delete the correct range, and remove a now
redundant check from insert_file_space.

Fixes: fe341eb151ec ("xfs: ensure that fpunch, fcollapse, and finsert operations are aligned to rt extent size")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

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