LU-5270 obdclass: do not lock repetitively when geting jobid

LU-5270 obdclass: do not lock repetitively when geting jobid

When trying to get customized job ID, cfs_get_environ() tries to
lock/unlock mmap_sem for two times, which is unnecessary.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: Iae8a45d87508e3c465c0424d5fa54e294d4fb2b1
Reviewed-on: http://review.whamcloud.com/10890
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>