ptrace: Convert ptrace_attach() to use lock guards

Created as testing for the conditional guard infrastructure.
Specifically this makes use of the following form:

  scoped_cond_guard (mutex_intr, return -ERESTARTNOINTR,
		     &task->signal->cred_guard_mutex) {
    ...
  }
  ...
  return 0;

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lkml.kernel.org/r/20231102110706.568467727%40infradead.org