oomd: rename last_hit_mem_pressure_limit -> mem_pressure_limit_hit_start
    
    Since this is only changed the first time the limit is hit (and remains
    set as long as the pressure remains over), I changed the name to better
    reflect that.
    
    Keeps consistent with "last_had_mem_reclaim" which is actually updated
    every time there is reclaim activity.
    
    (cherry picked from commit 69c8f0255a0a8127548907028bef09ea66056f9f)