nvmf: Schedule requests across multiple I/O queues

Similar to nvme(4), use the current CPU to select which I/O queue to
use. The assignment in nvmf_attach() had to be moved down since
sc->num_io_queues is initialized in nvmf_establish_connection().

Note that nvmecontrol(8) still defaults to using a single I/O queue
for an association.

Sponsored by: Chelsio Communications