nvmf: Handle shutdowns more gracefully

If an association is disconnected during a clean shutdown, abort all
pending and future I/O requests with an error to avoid hangs either due
to filesystem unmounts or a stuck GEOM event.

If an association is connected during a clean shutdown, gracefully
disconnect from the remote controller and close the open queues.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D45462