coredump: fix the setting of PF_DUMPCORE

Commit 079148b9 ("coredump: factor out the setting of PF_DUMPCORE")
cleaned up the setting of PF_DUMPCORE by removing it from all the
linux_binfmt->core_dump() and moving it to zap_threads().But this ended
up clearing all the previously set flags.  This causes issues during
core generation when tsk->flags is checked again (eg.  for PF_USED_MATH
to dump floating point registers).  Fix this.
Signed-off-by: Silesh C V <svellattu@mvista.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Mandeep Singh Baines <msb@chromium.org>
Cc: <stable@vger.kernel.org>	[3.10+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/coredump.c

@@ -306,7 +306,7 @@ static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 	if (unlikely(nr < 0))
 		return nr;
 
-	tsk->flags = PF_DUMPCORE;
+	tsk->flags |= PF_DUMPCORE;
 	if (atomic_read(&mm->mm_users) == nr + 1)
 		goto done;
 	/*