Before commit 47e65328, next_thread() took
a const task_t.  Reinstate the const qualifier, getting the next thread
never changes the current thread.
Signed-off-by: NKeith Owens <kaos@sgi.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>