nvme: enable to inject errors into admin commands

This enables to inject errors into the commands submitted to the admin
queue.

It is useful to test error handling in the controller initialization.

	# echo 100 > /sys/kernel/debug/nvme0/fault_inject/probability
	# echo 1 > /sys/kernel/debug/nvme0/fault_inject/times
	# echo 10 > /sys/kernel/debug/nvme0/fault_inject/space
	# nvme reset /dev/nvme0
	# dmesg
	...
	nvme nvme0: Could not set queue count (16385)
	nvme nvme0: IO queues not created

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>