IB/srp: Add more logging

Log sgid and dgid when reporting that a login has been rejected or when
a host has been added. This makes it easy to figure out which initiator
and target ports these messages apply to.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>