powerpc/mm/hash: Pull hash constants into hash__alloc_context_id()

The min and max context id values used in alloc_context_id() are
currently the right values for use on hash, and happen to also be safe
for use on radix.

But we need to change that in a subsequent patch, so make the min/max
ids parameters and pull the hash values into hsah__alloc_context_id().

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