freezer: do not send a fake signal to a PF_DUMPCORE thread

freezer: do not send a fake signal to a PF_DUMPCORE thread

A coredumping thread can't be frozen anyway but the fake signal sent by
freeze_task() can confuse dump_write/wait_for_dump_helpers/etc and
interrupt the coredump.

We are going to make the do_coredump() paths freezable but the fake
TIF_SIGPENDING doesn't help, it only makes sense when we assume that the
target can return to user-mode and call get_signal_to_deliver().

Change freeze_task() to check PF_DUMPCORE along with PF_KTHREAD.  We need
to recheck PF_DUMPCORE under ->siglock to avoid the race with
zap_threads() which can set this flag right before we take the lock.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Mandeep Singh Baines <msb@chromium.org>
Cc: Neil Horman <nhorman@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>