socket: remove fanout support when using older linux kernels

[Matthiasfranck]

In https://github.com/jow-/ucode/commit/0662de64bd1f61c2bba8792438fee8da0c706889 the fanout_args struct is used, which is only availble on linux kernels >= 5.11 (https://github.com/torvalds/linux/commit> This commit adds a check and does not compile the fanout pieces when on kernels < 5.11.