[SCSI] fcoe: move offload exchange manager pointer from fcoe_port to fcoe_interface

The offload EM pointer is only used when setting up a new libfc instance, but
as it's designed to be shared among NPIV VN_Ports it should be tracked in
fcoe_interface.

With the host-list changed to track fcoe_interfaces as well, this is needed
before we can remove the priv pointer from that structure (which is only there
to help in the transition, and stops making sense once NPIV is enabled).

Signed-off-by: Chris Leech <christopher.leech@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>