zbd: Rename zbd_init()

zbd: Rename zbd_init()

Clarify the execution context of zbd_init() by renaming this function
to zbd_setup_files() as it is called from the setup_files() function.
While at it, wrap the use of zbd_free_zone_info() into the inline
function zbd_close_file() to avoid an unecessary function call when
closing files that are not zoned block device files of zonemode=zbd
jobs, that is, files that do not have zbd_info initialized.

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