seccomp: add the synchronous mode for seccomp_unotify

seccomp: add the synchronous mode for seccomp_unotify

seccomp_unotify allows more privileged processes do actions on behalf
of less privileged processes.

In many cases, the workflow is fully synchronous. It means a target
process triggers a system call and passes controls to a supervisor
process that handles the system call and returns controls to the target
process. In this context, "synchronous" means that only one process is
running and another one is waiting.

There is the WF_CURRENT_CPU flag that is used to advise the scheduler to
move the wakee to the current CPU. For such synchronous workflows, it
makes context switches a few times faster.

Right now, each interaction takes 12µs. With this patch, it takes about
3µs.

This change introduce the SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP flag that
it used to enable the sync mode.

Signed-off-by: Andrei Vagin <avagin@google.com>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230308073201.3102738-5-avagin@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>