MIPS: Fix PKMAP with 32-bit MIPS huge page support

When 32-bit MIPS huge page support is enabled, we halve the number of
pointers a PTE page holds, making its last half go to waste.
Correspondingly, we should halve the number of kmap entries, as we just
initialized only a single pte table for that in pagetable_init().

Fixes: 35476311

 ("MIPS: Add partial 32-bit huge page support")
Signed-off-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>