staging/lustre/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>
Reviewed-on: http://review.whamcloud.com/10062
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4629


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>