LU-3467 ptlrpc: initialize request session early

LU-3467 ptlrpc: initialize request session early

Initialize request session early to make it available in
high-priority handlers

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: Ic4f5967d3ba8e723bf90accf20742cb4c6bc801c
Reviewed-on: http://review.whamcloud.com/7350
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>