mon: fix off-by-one in check for when sync falls behind

This is what e213b1bc25a212ffe42623c1d4b4eadf9f69319e intended to do
but managed to bungle by using >= instead of >.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>