task_work: Simplify the usage in ptrace_notify() and get_signal_to_deliver()

ptrace_notify() and get_signal_to_deliver() do unnecessary things
before task_work_run():

1. smp_mb__after_clear_bit() is not needed, test_and_clear_bit()
   implies mb().

2. And we do not need the barrier at all, in this case we only
   care about the "synchronous" works added by the task itself.

3. No need to clear TIF_NOTIFY_RESUME, and we should not assume
   task_works is the only user of this flag.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20120826191217.GA4238@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>