btrfs: zoned: factor out prepare_allocation_zoned()

Factor out prepare_allocation_zoned() for further extension. While at
it, optimize the if-branch a bit.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>