Fix "flexible array member X not at end of struct" compiling with GCC-11

[maurimo]

Fix a problem compiling with GCC-11 (present when I compiled latest Torch which includes gloo). The problem was also reported on Debian's ML, see for full context:
https://www.mail-archive.com/[email protected]/msg1875386.html

This problem should not be occurring in the first place and it's probably a bug of GCC, but luckily the workaround is quite simple too.

[Flamefire]

This is indeed correct while #346 is wrong. Needs rebase/Fix after that got already merged though.

CC @kumpera

[Flamefire]

IMO this is technically a strict aliasing violation and hence UB. At least buf needs to be aligned as struct ethtool_link_settings and possibly a placement new needs to be used.

Idea: Using an union to enforce the size might work:

  constexpr auto bufsize = sizeof(struct ethtool_link_settings) + sizeof(__u32)*link_mode_data_nwords;
  union {
    struct ethtool_link_settings req;
    char buf[bufsize];
  } ecmd;