The Offspring seem like two different bands: One's a passel of punk rockers specializing in gallop-tempo anthems; the other's a bunch of wiseasses who traffic in clever novelty-rock tunes. In the second incarnation, they're pretty fly, but when they play it straight they sound too much like, well, white guys. Splinter, album No. 7 from these Left Coasters, features a few snarky belly busters, like ''The Worst Hangover Ever'' and ''When You're in Prison,'' and rather too much of the straight-ahead stuff. Sure, the Offspring pack a lot of punch, but you sorta find yourself wishing they'd give birth to a pure comedy CD just once.