llist: Add nonatomic __llist_add() and __llist_dell_all()

We'll use these in the new, lockless kretprobes code.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/159870619318.1229682.13027387548510028721.stgit@devnote2