zbd: create a local zbdi variable for f->zbd_info

zbd: create a local zbdi variable for f->zbd_info

Create a local zbdi variable for f->zbd_info for the following functions:
zbd_open_zone(), zbd_convert_to_open_zone(), and zbd_adjust_block().

This avoids unnecessary indirections.

No functional change intended.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>