staging: zram: protect zram_reset_device() call

Commit 9b3bb7abcdf2df0f1b2657e6cbc9d06bc2b3b36f (remove
zram_sysfs file (v2)) accidentally made zram_reset_device()
racy. Protect zram_reset_device() call with zram->lock.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Jerome Marchand <jmarchand@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zram/zram_drv.c

@@ -527,8 +527,11 @@ static void zram_reset_device(struct zram *zram)
 	size_t index;
 	struct zram_meta *meta;
 
-	if (!zram->init_done)
+	down_write(&zram->init_lock);
+	if (!zram->init_done) {
+		up_write(&zram->init_lock);
 		return;
+	}
 
 	meta = zram->meta;
 	zram->init_done = 0;
@@ -549,6 +552,7 @@ static void zram_reset_device(struct zram *zram)
 
 	zram->disksize = 0;
 	set_capacity(zram->disk, 0);
+	up_write(&zram->init_lock);
 }
 
 static void zram_init_device(struct zram *zram, struct zram_meta *meta)