selftests/bpf: add test for bpf_msg_pop_data() overflow

selftests/bpf: add test for bpf_msg_pop_data() overflow

Add a test in sockmap_basic.c that calls bpf_msg_pop_data() with a length
close to U32_MAX, which overflows the start + len bounds check. The sk_msg
program records the return value over a sendmsg and the test checks that
the call is rejected with -EINVAL.

Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Sechang Lim <rhkrqnwk98@gmail.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/r/20260615021959.140010-7-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>