libext2fs: try to always use PUNCH_HOLE for unix_discard

libext2fs: try to always use PUNCH_HOLE for unix_discard

Now that block devices support PUNCH_HOLE via fallocate, refactor the
unix_discard code into a helper that will always try to use it.  For
block devices we can fall back to BLKDISCARD, but we prefer to use
fallocate because it will always invalidate the page cache of the zeroed
region.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>