[SCSI] libfc: add hook for FC-4 provider registration

Allow FC-4 provider modules to hook into libfc, mostly for targets.
This should allow any FC-4 module to handle PRLI requests and maintain
process-association states.

Each provider registers its ops with libfc and then will be called for
any incoming PRLI for that FC-4 type on any instance.   The provider
can decide whether to handle that particular instance using any method
it likes, such as ACLs or other configuration information.

A count is kept of the number of successful PRLIs from the remote port.
Providers are called back with an implicit PRLO when the remote port
is about to be deleted or has been reset.

fc_lport_recv_req() now sends incoming FC-4 requests to FC-4 providers,
and there is a built-in provider always registered for handling
incoming ELS requests.

The call to provider recv() routines uses rcu_read_lock()
so that providers aren't removed during the call.  That lock is very
cheap and shouldn't affect any performance on ELS requests.
Providers can rely on the RCU lock to protect a session lookup as well.

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>