UPSTREAM: zram: avoid kunmap_atomic() of a NULL pointer

(cherry picked from commit c406515239376fc93a30d5d03192182160cbd3fb)

zram could kunmap_atomic() a NULL pointer in a rare situation: a zram
page becomes a full-zeroed page after a partial write io.  The current
code doesn't handle this case and performs kunmap_atomic() on a NULL
pointer, which panics the kernel.

This patch fixes this issue.

Change-Id: I6358b55555328a97b1b9004629b87044c0e27fdd
Signed-off-by: Weijie Yang <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Weijie Yang <[email protected]>
Acked-by: Jerome Marchand <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
(cherry picked from commit ba922d3)

Author: Weijie Yang

drivers/block/zram/zram_drv.c

@@ -485,7 +485,8 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 	}
 
 	if (page_zero_filled(uncmem)) {
-		kunmap_atomic(user_mem);
+		if (user_mem)
+			kunmap_atomic(user_mem);
 		/* Free memory associated with this sector now. */
 		bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
 		zram_free_page(zram, index);