zbd: do not set zbd handlers for conventional zones

zbd: do not set zbd handlers for conventional zones

When zbd_adjust_block() modifies io_u to satisfy write pointer
restrictions, it may change the zone for the io_u. The function sets
pointers to zbd_queue_io() and zbd_put_io() handlers to io_u to
further process write pointer zones. However, when the I/O is
redirected to a conventional zone, these handlers should not
be set in io_u.

Skip setting the handlers when this function returns a conventional
zone. When zbd_adjust_block() can not find a zone to fit the I/O,
the existing code unlocks the zone pointer 'zb' used in the function.
This unlock should not be performed if 'zb' points to a conventional
zone upon return, skip it in this case.

These changes make the assert for 'zb' pointer near 'accept' label in
zbd_adjust_block() unnecessary. Replace it with assert for zb->has_wp,
since the zone at the step shall have write pointer.

Since zone locking functions (zone_lock(), zbd_queue_io() and
zbd_put_io()) are supposed to be called only for write pointer zones,
add assertions to zone_lock() and zone_unlock() to make sure this is
the case. This allows us to convert a few existing conditional checks
to assertions to make zone type validation more strict.

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