nvme: synchronize access to ctrl->namespaces

nvme: synchronize access to ctrl->namespaces

Currently traversal and modification of ctrl->namespaces happens completely
unsynchronized, which can be fixed by the addition of a simple mutex.

Note: nvme_dev_ioctl will be handled in the next patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 69d3b8ac15a5eb938e6a01909f6cc8ae4b5d3a17)

Orabug: 24583236
Signed-off-by: Ashok Vairavan <ashok.vairavan@oracle.com>