nvme: fixes for NVME_IOCTL_IO_CMD on the char device

nvme: fixes for NVME_IOCTL_IO_CMD on the char device

BugLink: http://bugs.launchpad.net/bugs/1531539
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>
(cherry picked from commit bfd8947194b2e2a53db82bbc7eb7c15d028c46db)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>