Add 'stop' subcommand to kublk utility that uses the new
UBLK_CMD_TRY_STOP_DEV command when --safe option is specified.
This allows stopping a device only if it has no active openers,
returning -EBUSY otherwise.
Also add test_generic_16.sh to test the new functionality.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>