LU-6695 jobstats: skip too long env variables

LU-6695 jobstats: skip too long env variables

The PATH env variable can be very large, it may exceed the page
size sometimes. cfs_get_envrion() should skip such too long env
variables instead of returning error.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I43c103a647cde3afa499dec3a11312155c3cf924
Reviewed-on: http://review.whamcloud.com/15177
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>