MIPS: Compat: Zero upper 32-bit of offset_high and offset_low.

Through sys_llseek() arguably should do exactly that it doesn't which
means llseek(2) will fail for o32 processes if offset_low has bit 31 set.

As suggested by Heiko Carstens.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>