net: rds: Simplify the allocation of slab caches in rds_conn_init

Use the new KMEM_CACHE() macro instead of direct kmem_cache_create
to simplify the creation of SLAB caches.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20240124075801.471330-1-chentao@kylinos.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>