LU-6527 ext4: journal_commit_callback optimization

LU-6527 ext4: journal_commit_callback optimization

Remove spin lock/unlock s_md_lock - it gives too
much overhead with huge transactions. This place
is not racy because txn is closed to the moment
when callbacks are called.

Change-Id: Ieaf76cce52854b9e5481c729bbb539eb7f97213f
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@seagate.com>
Xyratex-bug-id: MRP-2466
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/5690
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@seagate.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@seagate.com>
Tested-by: Jenkins
Tested-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-on: http://review.whamcloud.com/14610
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>