MIPS: Handle address errors for accesses above CPU max virtual user address

Address errors have always been treated as unaliged accesses and handled
as such. But address errors are also issued for illegal accesses like
user to kernel space or accesses outside of implemented spaces. This
change implements Linux exception handling for accesses to the illegal
space above the CPU implemented maximum virtual user address and the
MIPS 64bit architecture maximum. With this we can now use a fixed value
for the maximum task size on every MIPS CPU and get a more optimized
access_ok().

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>