powerpc: Zero fill the return values of rtas argument buffer

The kernel copy of the rtas args struct contains the return
value(s) for the specified rtas call.  These are copied back
to user space with the assumption that every value has been
set by the rtas call, which turns out to be not always true.
Thus userspace can see random values and think the call failed
when in fact it succeeded, but for some reason didn't set one
of the return values.

This fixes the problem by zeroing out the return value fields
of the rtas args struct before processing the rtas call.

Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>