oomd: don't get pressure candidates on every interval
    
    Only start collecting candidates for a memory pressure kill when we're
    hitting the limit (but before the duration hitting that limit is
    exceeded). This brings CPU util from ~1% to 0.3%.
    
    Addresses CPU util from
    https://bugzilla.redhat.com/show_bug.cgi?id=1941340
    and
    https://bugzilla.redhat.com/show_bug.cgi?id=1944646
    
    (cherry picked from commit cb13961ada52c1b27f6d6c2c6e37a2901f01ed30)