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: WANG Cong <xiyou.wangcong@gmail.com>

Add sg_init_table() call as well.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>