LU-4629 ptlrpc: fix NULL pointer dereference of {exp,imp}_obd

LU-4629 ptlrpc: fix NULL pointer dereference of {exp,imp}_obd

Pointer 'obd' checked for NULL at line 694 may be dereferenced at
line 813.

Pointer 'req->rq_export->exp_obd' checked for NULL at line 1155
may be dereferenced at line 1164. Also there is one similar error
on line 1170.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I4e0d40bb634415a3f7f1a38f66139b89b9f97772
Reviewed-on: http://review.whamcloud.com/10062
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>