Move bpf_prog_sub to be exported by compat

Move bpf_prog_sub to be exported by compat

Having bpf_prog_sub defined in the header file is causing compilation
issues on RHEL-7.6, since this OS does not really support XDP, it just
has only an empty implementation of it, so some parts can be missing.

The #include <linux/filter.h> caused the build issues, so we move the
backport to a compat C file and export it there, similar to upstream
where they also implement this function in a C file (kernel/bpf/syscall.c).

Signed-off-by: Vladimir Sokolovsky <vlad@mellanox.com>