binder: tell userspace to dump current backtrace when detected oneway spamming

When async binder buffer got exhausted, some normal oneway transactions
will also be discarded and may cause system or application failures. By
that time, the binder debug information we dump may not be relevant to
the root cause. And this issue is difficult to debug if without the
backtrace of the thread sending spam.

This change will send BR_ONEWAY_SPAM_SUSPECT to userspace when oneway
spamming is detected, request to dump current backtrace. Oneway spamming
will be reported only once when exceeding the threshold (target process
dips below 80% of its oneway space, and current process is responsible for
either more than 50 transactions, or more than 50% of the oneway space).
And the detection will restart when the async buffer has returned to a
healthy state.

Acked-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Hang Lu <hangl@codeaurora.org>
Link: https://lore.kernel.org/r/1617961246-4502-3-git-send-email-hangl@codeaurora.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>