LU-4413 osp: move seq allocation out of osp_import_event

Because seq allocation(osp_init_pre_fid) might be stuck
during RPC, move it out of osp_import_event, which is
inside ptlrpcd_rcv. Otherwise, some other import RPCs(like
connect req)might be blocked in ptlrpcd_rcv.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Ib4014f8b0088ea3613fa4d53d3e274f5bdfe70c7
Reviewed-on: http://review.whamcloud.com/8997
Tested-by: Jenkins
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>