ptrace: fix exit_ptrace() vs ptrace_traceme() race

Pointed out by Roland.  The bug was recently introduced by me in
"forget_original_parent: split out the un-ptrace part", commit
39c626ae.

Since that patch we have a window after exit_ptrace() drops tasklist and
before forget_original_parent() takes it again.  In this window the child
can do ptrace(PTRACE_TRACEME) and nobody can untrace this child after
that.

Change ptrace_traceme() to not attach to the exiting ->real_parent.  We
don't report the error in this case, we pretend we attach right before
->real_parent calls exit_ptrace() which should untrace us anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>