powerpc/booke64: Move mb() to __set_pte_at() with kernel-addr test

map_kernel() doesn't catch all places that create kernel PTEs.  In
particular, vmalloc() calls set_pte_at() directly.  This causes a
crash when booting a non-SMP kernel on e6500.

Move the sync to __set_pte(), to be executed only for kernel addresses.

Signed-off-by: Scott Wood <scottwood@freescale.com>