Add map parameter to prog load which will allow reuse of existing
maps instead of creating new ones.

We need feature detection and compat code for reallocarray, since
it's not available in many libc versions.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NQuentin Monnet <quentin.monnet@netronome.com>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>