nvme-fabrics: Add FC LLDD loopback driver to test FC-NVME

Add FC LLDD loopback driver to test FC host and target transport within
nvme-fabrics

To aid in the development and testing of the lower-level api of the FC
transport, this loopback driver has been created to act as if it were a
FC hba driver supporting both the host interfaces as well as the target
interfaces with the nvme FC transport.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Jay Freyensee <james_p_freyensee@linux.intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>