[MIPS] Fix USER_PTRS_PER_PGD for 64K page size.

The code in pgtable-64.h assumes TASK_SIZE is always bigger than a first
level PGDIR_SIZE. This is not the case for 64K pages, where task size is
40 bits (1TB) and a pgd entry can map 42 bits. This leads to
USER_PTRS_PER_PGD being zero for 64K pages.

Signed-off-by: Peter Watkins <treestem@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>