zbd: move and cleanup code

zbd: move and cleanup code

Move zone manipulation helper functions at the beginning of the zbd.c
file to avoid forward declarations and to group these functions
together apart from the IO manipulation functions.
Also fix function comments.

No functional changes.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20211214012413.464798-4-damien.lemoal@opensource.wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>