MIPS: compat: Don't clobber personality bits in 32-bit sys_personality().

If PER_LINUX32 has been set on a 32-bit kernel, only twiddle with the
low-order personality bits, let the upper bits pass through.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Cc: Camm Maguire <camm@maguirefamily.org>
Patchwork: https://patchwork.linux-mips.org/patch/1751/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>