nvmet: support reservation feature

This patch implements the reservation feature, including:
  1. reservation register(register, unregister and replace).
  2. reservation acquire(acquire, preempt, preempt and abort).
  3. reservation release(release and clear).
  4. reservation report.
  5. set feature and get feature of reservation notify mask.
  6. get log page of reservation event.

Not supported:
  1. persistent reservation through power loss.

Test cases:
  Use nvme-cli and fio to test all implemented sub features:
  1. use nvme resv-register to register host a registrant or
     unregister or replace a new key.
  2. use nvme resv-acquire to set host to the holder, and use fio
     to send read and write io in all reservation type. And also
     test preempt and "preempt and abort".
  3. use nvme resv-report to show all registrants and reservation
     status.
  4. use nvme resv-release to release all registrants.
  5. use nvme get-log to get events generated by the preceding
     operations.

In addition, make reservation configurable, one can set ns to
support reservation before enable ns. The default of resv_enable
is false.

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Reviewed-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>