powerpc/mm/slb: Move comment next to the code it's referring to

powerpc/mm/slb: Move comment next to the code it's referring to

There is a comment in slb_allocate() referring to the load of
paca->vmalloc_sllp, but it's several lines prior in the assembly.
We're about to change this code, and we want to add another comment,
so move the comment immediately prior to the instruction it's talking
about.

Signed-off-by: Michael Ellerman <[email protected]>
Reviewed-by: Aneesh Kumar K.V <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>