Documentation: ublk: document UBLK_F_USER_RECOVERY_FAIL_IO

- user recovery feature description

  Two new features are added for user recovery: ``UBLK_F_USER_RECOVERY`` and

  ``UBLK_F_USER_RECOVERY_REISSUE``.

  With ``UBLK_F_USER_RECOVERY`` set, after one ubq_daemon(ublk server's io

  Three new features are added for user recovery: ``UBLK_F_USER_RECOVERY``,

  ``UBLK_F_USER_RECOVERY_REISSUE``, and ``UBLK_F_USER_RECOVERY_FAIL_IO``. To

  enable recovery of ublk devices after the ublk server exits, the ublk server

  should specify the ``UBLK_F_USER_RECOVERY`` flag when creating the device. The

  ublk server may additionally specify at most one of

  ``UBLK_F_USER_RECOVERY_REISSUE`` and ``UBLK_F_USER_RECOVERY_FAIL_IO`` to

  modify how I/O is handled while the ublk server is dying/dead (this is called

  the ``nosrv`` case in the driver code).

  With just ``UBLK_F_USER_RECOVERY`` set, after one ubq_daemon(ublk server's io

  handler) is dying, ublk does not delete ``/dev/ublkb*`` during the whole

  recovery stage and ublk device ID is kept. It is ublk server's

  responsibility to recover the device context by its own knowledge.

  Requests which have not been issued to userspace are requeued. Requests

  which have been issued to userspace are aborted.

  With ``UBLK_F_USER_RECOVERY_REISSUE`` set, after one ubq_daemon(ublk

  server's io handler) is dying, contrary to ``UBLK_F_USER_RECOVERY``,

  With ``UBLK_F_USER_RECOVERY_REISSUE`` additionally set, after one ubq_daemon

  (ublk server's io handler) is dying, contrary to ``UBLK_F_USER_RECOVERY``,

  requests which have been issued to userspace are requeued and will be

  re-issued to the new process after handling ``UBLK_CMD_END_USER_RECOVERY``.

  ``UBLK_F_USER_RECOVERY_REISSUE`` is designed for backends who tolerate

  double-write since the driver may issue the same I/O request twice. It

  might be useful to a read-only FS or a VM backend.

  With ``UBLK_F_USER_RECOVERY_FAIL_IO`` additionally set, after the ublk server

  exits, requests which have issued to userspace are failed, as are any

  subsequently issued requests. Applications continuously issuing I/O against

  devices with this flag set will see a stream of I/O errors until a new ublk

  server recovers the device.

Unprivileged ublk device is supported by passing ``UBLK_F_UNPRIVILEGED_DEV``.

Once the flag is set, all control commands can be sent by unprivileged

user. Except for command of ``UBLK_CMD_ADD_DEV``, permission check on