sysrq: use SEND_SIG_FORCED instead of force_sig()

Change send_sig_all() to use do_send_sig_info(SEND_SIG_FORCED) instead
of force_sig(SIGKILL).  With the recent changes we do not need force_ to
kill the CLONE_NEWPID tasks.

And this is more correct.  force_sig() can race with the exiting thread,
while do_send_sig_info(group => true) kill the whole process.

Some more notes from Oleg Nesterov:

> Just one note. This change makes no difference for sysrq_handle_kill().
> But it obviously changes the behaviour sysrq_handle_term(). I think
> this is fine, if you want to really kill the task which blocks/ignores
> SIGTERM you can use sysrq_handle_kill().
>
> Even ignoring the reasons why force_sig() is simply wrong here,
> force_sig(SIGTERM) looks strange. The task won't be killed if it has
> a handler, but SIG_IGN can't help. However if it has the handler
> but blocks SIGTERM temporary (this is very common) it will be killed.

Also,

> force_sig() can't kill the process if the main thread has already
> exited. IOW, it is trivial to create the process which can't be
> killed by sysrq.

So, this patch fixes the issue.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>