punch-alternating: detect xfs realtime files with large allocation units

punch-alternating: detect xfs realtime files with large allocation units

For files on the XFS realtime volume, it's possible that the file
allocation unit (aka the minimum size we have to punch to deallocate
file blocks) could be greater than a single fs block.  This utility
assumed that it's always possible to punch a single fs block, but for
these types of files, all that does is zeroes the page cache.  While
that's what most *user applications* want, fstests uses punching to
fragment file mapping metadata and/or fragment free space, so adapt this
test for that purpose by detecting realtime files.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>