nvmf: Permit failing I/O requests while disconnected

Add a kern.nvmf.fail_on_disconnection sysctl similar to the
kern.iscsi.fail_on_disconnection sysctl. This causes pending I/O
requests to fail with an error if an association is disconnected
instead of requeueing to be retried once the association is
reconnected. As with iSCSI, the default is to queue and retry
operations.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D45308