xfs: test fallocate ops when rt extent size is and isn't a power of 2

xfs: test fallocate ops when rt extent size is and isn't a power of 2

Make sure that fallocate works when the rt extent size is and isn't a
power of 2.

[Eryu: require filefrag command, add _require_loop, use xfs_io to do
truncate]

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>