A seven-song EP recorded in Dave Grohl’s studio (he’s a longtime compatriot and, pre-Nirvana, a member), “Sessions” masters the tricky calculus of a punk-rock reunion: Sound the same as you used to, and it’s sad; sound different, and you might as well not have bothered. Songs like “Jamin at 606,” which seems to have been not so much recorded as shot out of a cannon, and the 1988-meets-2011 “Stopwatch” get the nostalgia/newness balance just right, paying homage to the band’s past without cannibalizing it.