LU-8998 tests: improve sanity.sh::check_seq_oid()

LU-8998 tests: improve sanity.sh::check_seq_oid()

Enable the use of debugfs to decode the OST "fid" xattr to
avoid an unmount/remount cycle of the OST if possible.

Improve sanity.sh::check_seq_oid() in test_27z to be agnostic
about field position, and improve overall code style to avoid
external command usage.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ia971e33cc3d8a5e4ca6f821116f12c0a72bcab07
Reviewed-on: https://review.whamcloud.com/26569
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>