Fix up 'need_resched()' definition

We should not go through the task pointer to get at the thread info,
since it's usually cheaper to just access the thread info directly.

So don't make the code look up 'current', when we can just use the
thread info accessor functions directly.  This generally avoids one
level of indirection and tends to work better together with code that
also looks at other thread flags (eg preempt_count).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>