exec: make de_thread() killable

Change de_thread() to use KILLABLE rather than UNINTERRUPTIBLE while
waiting for other threads.  The only complication is that we should
clear ->group_exit_task and ->notify_count before we return, and we
should do this under tasklist_lock.  -EAGAIN is used to match the
initial signal_group_exit() check/return, it doesn't really matter.

This fixes the (unlikely) race with coredump.  de_thread() checks
signal_group_exit() before it starts to kill the subthreads, but this
can't help if another CLONE_VM (but non CLONE_THREAD) task starts the
coredumping after de_thread() unlocks ->siglock.  In this case the
killed sub-thread can block in exit_mm() waiting for coredump_finish(),
execing thread waits for that sub-thead, and the coredumping thread
waits for execing thread.  Deadlock.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>