bcachefs: Convert snapshot table to RCU array

bcachefs: Convert snapshot table to RCU array

This switches the generic radix tree for the in-memory table of snapshot
nodes to a simple rcu array. This means we have to add new locking to
deal with reallocations, but is faster than traversing the radix tree.

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