MIPS: tlbex: Fix broken offsets on r2 without XPA

MIPS: tlbex: Fix broken offsets on r2 without XPA

Commit c5b367835cfc ("MIPS: Add support for XPA.") changed
build_pte_present() and build_pte_writable() to assume a constant offset
of _PAGE_READ and _PAGE_WRITE relative to _PAGE_PRESENT, however this is
no longer true for some MIPS32R2 builds since commit be0c37c985ed
("MIPS: Rearrange PTE bits into fixed positions.") which moved the
_PAGE_READ PTE bit away from the _PAGE_PRESENT bit, with the _PAGE_WRITE
bit falling into its place.

Make use of the _PAGE_READ and _PAGE_WRITE definitions to calculate the
correct mask to apply instead of hard coding 3 (for _PAGE_PRESENT |
_PAGE_READ) or 5 (for _PAGE_PRESENT | _PAGE_WRITE).

Fixes: c5b367835cfc ("MIPS: Add support for XPA.")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9888/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>