CLEANUP: fd: use a union in fd_rm_from_fd_list() to shut aliasing warnings

CLEANUP: fd: use a union in fd_rm_from_fd_list() to shut aliasing warnings

Enabling strict aliasing fails in fd.c when using the double-word CAS,
let's get rid of the (void**)(void*)&cur_list junk and use a union
instead. This way the compiler knows they do alias.