NVMe: Freeze admin queue on device failure

This fixes a race accessing an invalid address when a controller's admin
queue is in use during a reset for failure or hot removal occurs. The
admin queue will be frozen to prevent new users from entering prior to
the doorbell queue being unmapped.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>