powerpc: correct register usage in 64-bit syscall exit path

Since we don't restore the volatile registers in the syscall exit
path, we need to make sure we don't leak any potentially interesting
values from the kernel to userspace.  This was already the case for
all except r11.  This makes it use r11 for an MSR value, so r11 will
have an (uninteresting) MSR value in it on return to userspace.

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