parisc: factor out sid to protid conversion

Create a new __space_to_prot inline to convert the space id (mmu context)
to a protection id. Sadly it doesn't look like the #ifdef can be eliminated
since relying on the compiler to not truncate a bit on
	return (ctx >> SPACEID_SHIFT) << 1;
seems a little dodgy.

Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>