nvmf: Fail pass through commands while a controller is not associated

Previously this just dereferenced NULL qp pointers and panicked.
Instead, use a shared lock on the connection lock to protect access to
the qp pointers and allocate a request. If the controller is not
associated, fail the request with ECONNABORTED.

Possibly this should be honoring kern.nvmf.fail_on_disconnection and
block waiting for a reconnect request while disconnected if that
tunable is false.

Reported by: Suhas Lokesha <suhas@chelsio.com>
Sponsored by: Chelsio Communications