nvme: protect against simultaneous shutdown invocations

commit 77bf25ea70200cddf083f74b7f617e5f07fac8bd upstream.

[Back-ported to 4.4. The difference is the file location of the struct
definition that's adding the mutex.

This fixes reported kernel panics in 4.4-stable from simultaneous
controller resets that was never supposed to be allowed to happen.]

Signed-off-by: Keith Busch <keith.busch@intel.com>
[hch: split from a larger patch]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>