bcachefs: Rework lru btree

bcachefs: Rework lru btree

This patch changes how the LRU index works:

Instead of using KEY_TYPE_lru where the bucket the lru entry points to
is part of the value, this switches to KEY_TYPE_set and encoding the
bucket we refer to in the low bits of the key.

This means that we no longer have to check for collisions when inserting
LRU entries. We'll be making using of this in the next patch, which adds
a btree write buffer - a pure write buffer for btree updates, where
updates are appended to a simple array and then periodically sorted and
batch inserted.

This is a new on disk format version, and a forced upgrade.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>