ublk: Add UBLK_U_CMD_UPDATE_SIZE

ublk: Add UBLK_U_CMD_UPDATE_SIZE

Currently ublk only allows the size of the ublkb block device to be
set via UBLK_CMD_SET_PARAMS before UBLK_CMD_START_DEV is triggered.

This does not provide support for extendable user-space block devices
without having to stop and restart the underlying ublkb block device
causing IO interruption.

This patch adds a new ublk command UBLK_U_CMD_UPDATE_SIZE to allow the
ublk block device to be resized on-the-fly.

Feature flag UBLK_F_UPDATE_SIZE is also added to indicate support.

Signed-off-by: Omri Mann <omri@nvidia.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/2a370ab1-d85b-409d-b762-f9f3f6bdf705@nvidia.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>