nvme: introduce helper function to get ctrl state

The controller state is typically written by another CPU, so reading it
should ensure no optimizations are taken. This is a repeated pattern in
the driver, so start with adding a convenience function that returns the
controller state with READ_ONCE().

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>