zbd: simplify zone reset code

zbd: simplify zone reset code

zbd_reset_range() function is only called once from zbd_reset_zone()
and it is always called for an individual zone, not a range.

Make zone reset flow simpler by moving all the code needed
to reset a single zone from zbd_reset_range() to zbd_reset_zone().
Therefore, zbd_reset_range() is now dropped.

zbc_reset_zone() is always called with the zone already locked. Remove
recursive zone locking inside this function and add a note in the
description of this function saying that the caller is expected to have
the zone locked when calling it.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>