KVM: PPC: E500: Propagate errors when shadow mapping

When shadow mapping a page, mapping this page can fail. In that case we
don't have a shadow map.

Take this case into account, otherwise we might end up writing bogus TLB
entries into the host TLB.

While at it, also move the write_stlbe() calls into the respective TLBn
handlers.

Signed-off-by: Alexander Graf <agraf@suse.de>