nvme: release namespace SRCU protection before performing controller ioctls

Holding the SRCU critical section protecting the namespace list can
cause deadlocks when using the per-namespace admin passthrough ioctl to
delete as namespace.  Release it earlier when performing per-controller
ioctls to avoid that.

Reported-by: Kenneth Heitke <kenneth.heitke@intel.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>