zram: avoid null access when fail to alloc meta

zram_meta_alloc could fail so caller should check it.  Otherwise, your
system will hang.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -612,6 +612,8 @@ static ssize_t disksize_store(struct device *dev,
 
 	disksize = PAGE_ALIGN(disksize);
 	meta = zram_meta_alloc(disksize);
+	if (!meta)
+		return -ENOMEM;
 	down_write(&zram->init_lock);
 	if (zram->init_done) {
 		up_write(&zram->init_lock);