sched/eevdf: Re-arrange current protection in pick_eevdf()

The way pick_eevdf() limits preemption is by explicitly picking
current if it is still eligible. It does this without consideration of
the best in-tree task.

Move current protection to after the tree selection such that a follow
up patch can change the conditions.

Should be an semantics no-op at this point.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>