This adds a new btree which gets us a persistent per-snapshot-tree
identifier.

 - BTREE_ID_snapshot_trees
 - KEY_TYPE_snapshot_tree
 - bch_snapshot now has a field that points to a snapshot_tree

This is going to be used to designate one snapshot ID/subvolume out of a
given tree of snapshots as the "main" subvolume, so that we can do quota
accounting in that subvolume and not the rest.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>