selftests/bpf: Move macros to bpf_misc.h

Move the sizeof_field and offsetofend macros from individual test files
to the common bpf_misc.h to avoid duplication.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/97a3f3788bd3aec309100bc073a5c77130e371fd.1758094761.git.paul.chaignon@gmail.com