Now CONFIG_DEBUG_INITDATA is in, initial percpu data 
[__per_cpu_start,__per_cpu_end] can be declared as a redzone, and invalid 
accesses after boot can be detected, at least for i386.

We can let non possible cpus percpu data point to this 'redzone' instead of NULL .

NULL was not a good choice because part of [0..32768] memory may be readable 
and invalid accesses may happen unnoticed.

If CONFIG_DEBUG_INITDATA is not defined, each non possible cpu points to the 
initial percpu data (__per_cpu_offset[cpu] == 0), thus invalid accesses wont 
be detected/crash.

This patch also moves __per_cpu_offset[] to read_mostly area to avoid false 
sharing.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>