[SCSI] libfc: add discovery-private pointer for LLD

For VN_port to VN_port mode, FIP will do discovery and needs a
way to find its state from the local port or discovery structure.
It seems that any other LLD that implements its own discovery
would also need something like this.

Replace disc->lport with disc->priv, and use container_of to
find the lport.  We could use disc->priv for that, but
container_of is smaller and faster.

Signed-off-by: Joe Eykholt <jeykholt@cisco.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>