selftests/bpf: Fix unmap bug in prog_tests/xdp_metadata.c

selftests/bpf: Fix unmap bug in prog_tests/xdp_metadata.c

The function close_xsk() unmap via munmap() the wrong memory pointer.
The call xsk_umem__delete(xsk->umem) have already freed xsk->umem.
Thus the call to munmap(xsk->umem, UMEM_SIZE) will have unpredictable
behavior that can lead to Segmentation fault elsewhere, as man page
explain subsequent references to these pages will generate SIGSEGV.

Fixes: e2a46d54d7a1 ("selftests/bpf: Verify xdp_metadata xdp->af_xdp path")
Reported-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/167527517464.938135.13750760520577765269.stgit@firesoul