Alas, both POSIX and I got the fadvise() interface wrong.  It needs to take a
64-bit length, not a 32-bit one.  Because fadvise(POSIX_FADV_DONTNEED) on a
4TB file will require 1000 syscalls.  Silly.

There are glibc's in the wild which use the existing syscall, so we must
make a new one.