Move the file and rename internal BPF_SOCK_ADDR define to
BPF_SOCK_ADDR_STORE. This selftest will be extended in the next commit
with the wide loads.

Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>