selftests/bpf: Fix test_xdp_adjust_tail_grow2 selftest on powerpc

On powerpc cache line size is 128 bytes, so skb_shared_info must be
aligned accordingly.

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20250110103109.3670793-1-skb99@linux.ibm.com