cifsd: Select SG_POOL for SMB_SERVER_SMBDIRECT

cifsd: Select SG_POOL for SMB_SERVER_SMBDIRECT

hulk-robot following build error:
 fs/cifsd/transport_rdma.c: In function 'read_write_done':
 fs/cifsd/transport_rdma.c:1297:2: error: implicit declaration of
function 'sg_free_table_chained'
[-Werror=implicit-function-declaration]
  1297 |  sg_free_table_chained(&msg->sgt, SG_CHUNK_SIZE);

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

Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>