Patch from Russell King <rmk@arm.linux.org.uk>

show_task() attempts to calculate the amount of free space which hasn't been
written to on the kernel stack by reading from the base of the kernel stack
upwards.

However, it mistakenly uses the task_struct pointer as the base of the stack,
which it isn't, and this can cause an oops.

Here is a patch which uses the task thread pointer instead, which should be
located at the bottom of the kernel stack.  It appears this was missed when
the thread structure was introduced.