IB/srp: Export sgid to sysfs

On an initiator system with multiple IB ports it is not yet possible
to figure out what the originating port of an SRP connection is. Hence
make the source GID available in sysfs.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: David Dillow <dillowda@ornl.gov>
Signed-off-by: Roland Dreier <roland@purestorage.com>