nvme: fixes for NVME_IOCTL_IO_CMD on the char device
Make sure we synchronize access to the namespaces list and grab a reference
to the namespace before doing I/O. Make sure to reject the ioctl if multiple
namespaces are present as it's entirely unsafe, and warn when using it even
with a single namespace.
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>