IB/srpt: Use the source GID as session name

Use the source GID as session name instead of the initiator port ID
from the SRP login request. The only functional change in this patch
is that it changes the session name shown in debug messages.

Note: the fifth argument that is passed to target_alloc_session() is
what the SCSI target core uses as key for lookups in the ACL (access
control list) information.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>