btrfs: store index number instead of key in struct btrfs_delayed_item

All delayed items are for dir index keys, so there's really no point of
having an embedded struct btrfs_key in struct btrfs_delayed_item, which
makes the structure use more space than necessary (and adds a hole of 7
bytes).

So replace the key field with an index number (u64), which reduces the
size of struct btrfs_delayed_item from 112 bytes down to 96 bytes.

Some upcoming work will increase the structure size by 16 bytes, so this
change compensates for that future size increase.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>