[SCSI] libfc: do not log off rports before or after discovery

When receiving an RSCN, do not log off all rports.  This is
extremely disruptive.  If, after the GPN_FT response, some
rports haven't been listed, delete them.

Add field disc_id to structs fc_rport_priv and fc_disc.
disc_id is an arbitrary serial number used to identify the
rports found by the latest discovery.  This eliminates the need
to go through the rport list when restarting discovery.

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>