We need to use u64 because the future 64-bit ports can theoretically
return the same value for two different dentries, as pointed out by
Ulrich Weigand.

The patch also changes return value of the syscall to give length of
data copied, needed for valgrind support (this bit is by Philippe Elie).

Note this is not a complete fix for mixed 32/64: userspace needs to
figure out the kernel pointer size when reading from the buffer. But
that's another fix...

NOTE! any oprofile users will need to upgrade after this goes in, and
the user-space equivalent is checked into CVS.  Sorry for the inconvenience