nvme-fabrics: refactor queue ready check

Move the is_connected check to the fibre channel transport, as it has no
meaning for other transports.  To facilitate this split out a new
nvmf_fail_nonready_command helper that is called by the transport when
it is asked to handle a command on a queue that is not ready.

Also avoid a function call for the queue live fast path by inlining
the check.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Smart <james.smart@broadcom.com>