Pull the logic to get the "lock" for TLB shooting into

Pull the logic to get the "lock" for TLB shooting into
pmap_start_tlb_shoot(), to get the #ifdef goo out of
pmap_tlb_shoot{page,range,tlb}()

Use a normal do{}while(0) when nooping pmap_tlb_shootwait()

tweak and ok kettenis@