[POWERPC] Clear RI bit in MSR before restoring r13 when returning to userspace

Some instruction tracing tools use the RI (recoverable interrupt) bit
in the MSR to indicate when it's safe to single-step.  Currently we
clear RI after restoring r13 when returning to userspace.  However,
if we single-step past the point where r13 is restored, we'll corrupt
r13 in the exception entry code and not restore it.  This moves the
clearing of RI to just before r13 is restored so this doesn't happen.

Signed-off-by: Paul Mackerras <paulus@samba.org>