fix missing flags arg to fstatat syscall in fstat fallback path

fix missing flags arg to fstatat syscall in fstat fallback path

this code path is used only on archs without the plain, non-at
syscalls, and only when the fstat syscall fails with EBADF on a valid
file descriptor. this in turn can happen only for O_PATH file
descriptors, and may not happen at all on the newer kernels needed for
supporting such archs.

with the flags argument omitted, spurious fstat failures may happen
when the argument register happens to have the AT_SYMLINK_NOFOLLOW bit
set.