nvmet-fcloop: track ref counts for nports

A nport object is always used in association with targerport,
remoteport, tport and rport objects. Add explicit references for any of
the associated object. This ensures that nport is not removed too early
on shutdown sequences.

Signed-off-by: Daniel Wagner <wagi@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>