nvme: use srcu for iterating namespace list

nvme: use srcu for iterating namespace list

The nvme pci driver synchronizes with all the namespace queues during a
reset to ensure that there's no pending timeout work.

Meanwhile the timeout work potentially iterates those same namespaces to
freeze their queues.

Each of those namespace iterations use the same read lock. If a write
lock should somehow get between the synchronize and freeze steps, then
forward progress is deadlocked.

We had been relying on the nvme controller state machine to ensure the
reset work wouldn't conflict with timeout work. That guarantee may be a
bit fragile to rely on, so iterate the namespace lists without taking
potentially circular locks, as reported by lockdep.

Link: https://lore.kernel.org/all/20220930001943.zdbvolc3gkekfmcv@shindev/
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Tested-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>