ptrace_attach: shift send(SIGSTOP) into ptrace_set_stopped()

Turn send_sig_info(SIGSTOP) into send_signal_locked(SIGSTOP) and move it
from ptrace_attach() to ptrace_set_stopped().

This looks more logical and avoids lock(siglock) right after unlock().

Link: https://lkml.kernel.org/r/20240122171631.GA29844@redhat.com


Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>