ntdll: Ignore the hole in the FILE_PIPE_WAIT_FOR_BUFFER structure passed into NtFsControlFile before writing the structure to the server.

[Dan Kegel]

Rob's patch is the most precise solution.
The macro should probably go in a .h file as it might be
useful elsewhere.

Alternately I can suppress all errors from functions like
server_ioctl_file but it'd be better to just mark the holes
as defined.

Thanks, Rob!