arch/um/drivers/ubd_kern.c: fix a building error

Fix this uml building error:
arch/um/drivers/ubd_kern.c: In function 'do_ubd_request':
arch/um/drivers/ubd_kern.c:1118: error: implicit declaration of function
'sg_page'
arch/um/drivers/ubd_kern.c:1118: warning: passing argument 6 of
'prepare_request' makes pointer from integer without a cast
make[1]: *** [arch/um/drivers/ubd_kern.o] Error 1
make: *** [arch/um/drivers] Error 2
Signed-off-by: NWANG Cong <xiyou.wangcong@gmail.com>

Add sg_init_table() call as well.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

arch/um/drivers/ubd_kern.c

@@ -35,6 +35,7 @@
 #include "linux/genhd.h"
 #include "linux/spinlock.h"
 #include "linux/platform_device.h"
+#include "linux/scatterlist.h"
 #include "asm/segment.h"
 #include "asm/uaccess.h"
 #include "asm/irq.h"
@@ -704,6 +705,7 @@ static int ubd_add(int n, char **error_out)
 	ubd_dev->size = ROUND_BLOCK(ubd_dev->size);
 
 	INIT_LIST_HEAD(&ubd_dev->restart);
+	sg_init_table(&ubd_dev->sg, MAX_SG);
 
 	err = -ENOMEM;
 	ubd_dev->queue = blk_init_queue(do_ubd_request, &ubd_dev->lock);