RDMA/rxe: Support RDMA link creation and destruction per net namespace

RDMA/rxe: Support RDMA link creation and destruction per net namespace

After introducing dellink handling and per-net namespace management
for IPv4 and IPv6 sockets, extend rxe to create and destroy RDMA links
within each network namespace.

With this change, RDMA links can be instantiated both in init_net and
in other network namespaces. The lifecycle of the RDMA link is now tied
to the corresponding namespace and is properly cleaned up when the
namespace or link is removed.

This ensures rxe behaves correctly in multi-namespace environments and
keeps socket and RDMA link resources consistent across namespace
creation and teardown.

Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Link: https://patch.msgid.link/20260313023058.13020-4-yanjun.zhu@linux.dev
Signed-off-by: Leon Romanovsky <leon@kernel.org>