powerpc/mm/64s: Consolidate SLB assertions

The code for assert_slb_exists() and assert_slb_notexists() is almost
identical, except for the polarity of the WARN_ON(). In a future patch
we'll need to modify this code, so consolidate it now into a single
function.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>