v2 booke/kprobe: Fix stack corrupt issue when kprobe 'stwu'

[Tiejun Chen]

v1 -> v2: when allocate pgirq_ctx, use 'hw_cpu' to identify cpu ID in exc_lvl_early_init().

BTW, I already validated these patches on fsl mpc8536 by kprobe do_fork() and show_interrupts().

Note this bug I fixed is from another email thread, "[BUG?]3.0-rc4+ftrace+kprobe: 
set kprobe at instruction 'stwu' lead to system crash/freeze", Yong Zhang, <yong.zhang0 at gmail.com>
previously reported.

Tiejun