MIPS: mm: Be more explicit about PTE mode bit handling

MIPS: mm: Be more explicit about PTE mode bit handling

The XPA case in iPTE_SW or's in software mode bits to the pte_low value
(which is what actually ends up in the high 32 bits of EntryLo...). It
does this presuming that only bits in the upper 16 bits of the 32 bit
pte_low value will be set. Make this assumption explicit with a BUG_ON.

A similar assumption is made for the hardware mode bits, which are or'd
in with a single ori instruction. Make that assumption explicit with a
BUG_ON too.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13122/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>