[SCSI] libfc: Don't assume response request present.

Fix NULL pointer dereference crash occurs in fc_lport_bsg_request()
for bsg requests that do not contain a response request.
Specifically, FC_BSG_HST_ADD_RPORT and FC_BSG_HST_DEL_RPORT bsg
requests are not guaranteed to include a response request.

Signed-off-by: Hugh Daschbach <hdasch@broadcom.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>