IB/srpt: Accept GUIDs as port names

Port and ACL information must be configured before an initiator
logs in.  Make it possible to configure this information before
a subnet prefix has been assigned to a port by not only accepting
GIDs as target port and initiator port names but by also accepting
port GUIDs.

Add a 'priv' member to struct se_wwn to allow target drivers to
associate their own data with struct se_wwn.

Reported-by: Doug Ledford <dledford@redhat.com>
References: http://www.spinics.net/lists/linux-rdma/msg39505.html
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>