zonefs: introduce helper for zone management

Introduce a helper function for sending zone management commands to the
block device.

As zone management commands can change a zone write pointer position
reflected in the size of the zone file, this function expects the truncate
mutex to be held.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>