common: use fiemap to count extents and holes

common: use fiemap to count extents and holes

For the follwoing tests, this patch adds general script to get extent and
hole counts.

 xfs/137         (data vs filesize)
 xfs/138         (data vs filesize vs truncate)
 xfs/139         (data vs filesize vs partial truncate)
 xfs/140         (data vs filesize vs extending truncate)
 xfs/179         (data vs filesize w/ fsync)
 xfs/180         (data vs filesize w/ sync)
 xfs/182         (data vs filesize w/ recovery)

It also requires these tests to check for fiemap support.

[dchinner: use _require_xfs_io_command "fiemap" for consistency]

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>