ptrace: fix ptrace_unfreeze_traced() race with rt-lock

[ Upstream commit 0fdc91971b34cf6857b4cfd8c322ae936cfc189b ]

The patch "ptrace: fix ptrace vs tasklist_lock race" changed
ptrace_freeze_traced() to take task->saved_state into account, but
ptrace_unfreeze_traced() has the same problem and needs a similar fix:
it should check/update both ->state and ->saved_state.

Reported-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Fixes: "ptrace: fix ptrace vs tasklist_lock race"
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable-rt@vger.kernel.org
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>