nvme: add disk name to trace events

This will print the disk name to the nvme event trace for io requests so
a user can better distinguish traffic to different disks. This can be used
to  create disk based filters. For example, to see only nvme0n2 traffic:

  echo "disk == \"nvme0n2\"" > /sys/kernel/debug/tracing/events/nvme/filter

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
[hch: turned __assign_disk_name into an inline function]
Signed-off-by: Christoph Hellwig <hch@lst.de>