powerpc/64s: Rename slb_miss_realmode() to slb_miss_common()

slb_miss_realmode() doesn't always runs in real mode, which is what the
name implies. So rename it to avoid confusing people.

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