coredump: make wait_for_dump_helpers() freezable

coredump: make wait_for_dump_helpers() freezable

wait_for_dump_helpers() calls wake_up/kill_fasync from inside the
wait_event-like loop.  This is not needed and in fact this is not strictly
correct, we can/should do this only once after we change pipe->writers.
We could even check if it becomes zero.

With this change it is trivial to convert this code to use
wait_event_freezable() and make this function freezable/killable, only
SIGKILL can set TIF_SIGPENDING.

With this patch we check pipe->readers without pipe_lock(), this is fine.
Once we see pipe->readers == 1 we know that the handler decremented the
counter, this is all we need.

Note: wait_event_freezable() is "strange", perhaps it should be changed or
simply removed.  In the latter case we can change this code again to use
freezer_do_not_count + wait_event_interruptible.

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