selftests/bpf: Fix use-after-free in xdp_metadata test

selftests/bpf: Fix use-after-free in xdp_metadata test

ASAN reported a use-after-free in close_xsk().

The xsk->socket internally references xsk->umem via socket->ctx->umem,
so the socket must be deleted before the umem. Fix the order of
operations in close_xsk().

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-14-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>