btrfs: zoned: add ASSERT and WARN for EXTENT_BUFFER_ZONED_ZEROOUT handling

Add an ASSERT to catch a faulty delayed reference item resulting from
prematurely cleared extent buffer.

Also, add a WARN to detect if we try to dirty a ZEROOUT buffer again, which
is suspicious as its update will be lost.

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>