coredump: turn mm->core_startup_done into the pointer to struct core_state

mm->core_startup_done points to "struct completion startup_done" allocated
on the coredump_wait()'s stack.  Introduce the new structure, core_state,
which holds this "struct completion".  This way we can add more info
visible to the threads participating in coredump without enlarging
mm_struct.

No changes in affected .o files.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/exit.c b/kernel/exit.c
index 28a44a2..f7fa21d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -680,7 +680,7 @@
 		up_read(&mm->mmap_sem);
 		down_write(&mm->mmap_sem);
 		if (!--mm->core_waiters)
-			complete(mm->core_startup_done);
+			complete(&mm->core_state->startup);
 		up_write(&mm->mmap_sem);
 
 		wait_for_completion(&mm->core_done);