procfs: fix /proc/<pid>/stat stack pointer for kernel threads

Fix a small issue for the stack pointer in /proc/<pid>/stat.  In case of a
kernel thread the value of the printed stack pointer should be 0.

Signed-off-by: Stefani Seibold <stefani@seibold.net>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>