rbtree: provide rb_find_rcu() / rb_find_add_rcu()

Much like latch_tree, add two RCU methods for the regular RB-tree,
which can be used in conjunction with a seqcount to provide lockless
lookups.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20240903174603.3554182-7-andrii@kernel.org