powerpc/64s: SLB miss already has CTR saved for relocatable kernel

The EXCEPTION_PROLOG_1 used by SLB miss already saves CTR when the
kernel is built with CONFIG_RELOCATABLE. So it does not have to be
saved and reloaded when branching to slb_miss_realmode. It can be
restored from the PACA as usual.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>