uprobes: allow put_uprobe() from non-sleepable softirq context

Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which
makes it unsuitable to be called from more restricted context like softirq.

Let's make put_uprobe() agnostic to the context in which it is called,
and use work queue to defer the mutex-protected clean up steps.

RB tree removal step is also moved into work-deferred callback to avoid
potential deadlock between softirq-based timer callback, added in the
next patch, and the rest of uprobe code.

We can rework locking altogher as a follow up, but that's significantly
more tricky, so warrants its own patch set. For now, we need to make
sure that changes in the next patch that add timer thread work correctly
with existing approach, while concentrating on SRCU + timeout logic.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241024044159.3156646-2-andrii@kernel.org