btrfs: optimize out btrfs_is_zoned for !CONFIG_BLK_DEV_ZONED

Add an IS_ENABLED check for CONFIG_BLK_DEV_ZONED in addition to the
run-time check for the zone size.  This will allow to make use of
compiler dead code elimination for code guarded by btrfs_is_zoned, and
for example provide just a dangling prototype for a function instead of
adding a stub.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>