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: 5a1328d0

 ("block/rnbd-clt: Dynamically allocate sglist for rnbd_iu")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>