sign extension for 32bit syscalls on ppc64

[Paul Mackerras]

Stephen Rothwell writes:

> They do need to be sign extended for the new *at syscalls where you can pass
> a sepecial value AT_FDCWD (-10) to represent the current directory ...

So, compat_sys_openat() in fs/compat.c looks wrong to me then, if
AT_FDCWD is part of the ABI and not just an internal thing.  And I'm
now not sure whether some of the other *at syscalls do in fact need
compat wrappers...

Paul.