common: use _require_xfs_io_command() directly to check fiemap

common: use _require_xfs_io_command() directly to check fiemap

1) _require_fiemap and _require_xfs_io_command "fiemap" do the
   same thing, but some test cases use the former and some use
   the latter, so i feel they should be unified.

2) The number of helpers like this is slowly growing, but it's
   easy to simply use _require_xfs_io_command directly and just
   specify the command we want to check.

This is just a cleanup for keeping it simple.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>