madvise/fadvise 32-bit length

[Stefan <source@xxxxxxxxxxxxx>]

io_uring uses the __u32 len field in order to pass the length to madvise 
and fadvise, but these calls use an off_t, which is 64bit on 64bit 
platforms.

When using liburing, the length is silently truncated to 32bits (so 8GB 
length would become zero, which has a different meaning of "until the 
end of the file" for fadvise).

If my understanding is correct, we could fix this by introducing new 
operations MADVISE64 and FADVISE64, which use the addr3 field instead of 
the length field for length.