bcachefs: BTREE_ID_subvolume_children

Add a btree to record a parent -> child subvolume relationships,
according to the filesystem heirarchy.

The subvolume_children btree is a bitset btree: if a bit is set at pos
p, that means p.offset is a child of subvolume p.inode.

This will be used for efficiently listing subvolumes, as well as
recursive deletion.

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