selftests/bpf: Set CONFIG_PACKET=y for selftests

BPF selftest fails to build with below error:

CLNG-BPF [test_progs] lsm_cgroup.bpf.o
progs/lsm_cgroup.c:105:21: error: variable has incomplete type 'struct sockaddr_ll'
105 | struct sockaddr_ll sa = {};
| ^
progs/lsm_cgroup.c:105:9: note: forward declaration of 'struct sockaddr_ll'
105 | struct sockaddr_ll sa = {};
| ^
1 error generated.

lsm_cgroup selftest requires sockaddr_ll structure which is not there
in vmlinux.h when the kernel is built with CONFIG_PACKET=m.

Enabling CONFIG_PACKET=y ensures that sockaddr_ll is available in vmlinux,
allowing it to be captured in the generated vmlinux.h for bpf selftests.

Reported-by: Sachin P Bappalige <[e-mail]>
Signed-off-by: Saket Kumar Bhaskar <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/config

···
 CONFIG_IP6_NF_IPTABLES=y
 CONFIG_IP6_NF_FILTER=y
 CONFIG_NF_NAT=y
+CONFIG_PACKET=y
 CONFIG_RC_CORE=y
 CONFIG_SECURITY=y
 CONFIG_SECURITYFS=y