staging: android: ion: Bail out upon SIGKILL when allocating memory.

syzbot found that a thread can stall for minutes inside
ion_system_heap_allocate() after that thread was killed by SIGKILL [1].
Let's check for SIGKILL before doing memory allocation.

[1] https://syzkaller.appspot.com/bug?id=a0e3436829698d5824231251fad9d8e998f94f5eSigned-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: stable <stable@vger.kernel.org>
Reported-by: syzbot <syzbot+8ab2d0f39fb79fe6ca40@syzkaller.appspotmail.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://lore.kernel.org/r/d088f188-5f32-d8fc-b9a0-0b404f7501cc@I-love.SAKURA.ne.jpSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ion/ion_page_pool.c

@@ -8,11 +8,14 @@
 #include <linux/list.h>
 #include <linux/slab.h>
 #include <linux/swap.h>
+#include <linux/sched/signal.h>
 
 #include "ion.h"
 
 static inline struct page *ion_page_pool_alloc_pages(struct ion_page_pool *pool)
 {
+	if (fatal_signal_pending(current))
+		return NULL;
 	return alloc_pages(pool->gfp_mask, pool->order);
 }