zbd: introduce zone_unlock()

zbd: introduce zone_unlock()

ZBD code already defines a helper function to lock a device zone,
zone_lock(). There is no zone_unlock() function though.

Wrap zone mutex unlock to zone_unlock() helper along with an assert
to make sure that the unlock is successful, i.e. that the function
is being called with the pointer to a locked zone.

Suggested-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>