nvme: ensure subsystem reset is single threaded

The subsystem reset writes to a register, so we have to ensure the
device state is capable of handling that otherwise the driver may access
unmapped registers. Use the state machine to ensure the subsystem reset
doesn't try to write registers on a device already undergoing this type
of reset.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=214771
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
