We have to load the bigkernel second TLB entry on
secondary processors before we move over the use
the kernel trap table.  Otherwise we can take a
TLB miss somewhere in the post-4MB area and the
TLB handler is not prepared to service that.

The case that usually occurs is the prom_set_trap_table
call made by trampoline.S, since p1275buf usually sits
very near the end of the kernel image.  It worked by
luck most of the time as long as p1275buf sits within
a single page since earlier code running in trampoline.S
forced that TLB entry to be loaded by the OBP TLB miss
handler.

This was not fun to figure out.
Signed-off-by: David S. Miller <davem@davemloft.net>