net/af_xdp: revert use BPF link for XDP programs

net/af_xdp: revert use BPF link for XDP programs

The commit ae70cc6e893b ("net/af_xdp: use BPF link for XDP programs")
caused compilation errors on kernels older than v5.8 due to absence of
the bpf_link_info struct and some definitions in the linux/bpf.h header.
Since relying on the reported kernel version is not a robust solution
and also since there doesn't appear to be a suitable definition in the
bpf header that the preprocessor could rely on to determine support for
bpf link, we will take a different approach to solving the issue that
the original patch attempted to solve. The next commit will address
this.

Fixes: ae70cc6e893b ("net/af_xdp: use BPF link for XDP programs")

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>