block/rnbd: Select SG_POOL for RNBD_CLIENT

lkp reboot following build error:
drivers/block/rnbd/rnbd-clt.c: In function 'rnbd_softirq_done_fn':
>> drivers/block/rnbd/rnbd-clt.c:387:2: error: implicit declaration of function 'sg_free_table_chained' [-Werror=implicit-function-declaration]
387 | sg_free_table_chained(&iu->sgt, RNBD_INLINE_SG_CNT);
| ^~~~~~~~~~~~~~~~~~~~~

The reason is CONFIG_SG_POOL is not enabled in the config, to
avoid such failure, select SG_POOL in Kconfig for RNBD_CLIENT.

Fixes: 5a1328d0c3a7 ("block/rnbd-clt: Dynamically allocate sglist for rnbd_iu")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Jack Wang <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/rnbd/Kconfig

···
 	tristate "RDMA Network Block Device driver client"
 	depends on INFINIBAND_RTRS_CLIENT
 	select BLK_DEV_RNBD
+	select SG_POOL
 	help
 	  RNBD client is a network block device driver using rdma transport.