MIPS: Don't clobber personality high bits.

The high bits of current->personality carry settings that we don't want to
clobber on each exec.  Only clobber them if the lower bits that indicate
either PER_LINUX or PER_LINUX32 are invalid.

The clobbering prevents us from using useful bits like ADDR_NO_RANDOMIZE.

Reported-by: Camm Maguire <camm@maguirefamily.org>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Cc: Camm Maguire <camm@maguirefamily.org>
Patchwork: https://patchwork.linux-mips.org/patch/1750/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>