Kurt Cobain’s guitar from the “Smells Like Teen Spirit” music video will be sold at a Midtown auction next spring — and it’s expected to fetch millions, The Post has learned. The Nirvana frontman’s 1969 Fender Mustang will be put up as part of massive sale of the late Indianapolis Colts owner Jim Irsay’s music memorabilia by Christie’s in March, with an estimated sale price of between $2.5 million and $5 million, according to the company’s global president Alexander Rotter. The sale is set to bring a record number of music fans into the free-to-enter Rockefeller Plaza galleries, which previously peddled guitars that once belonged to rockers Mark Knopfler, Eric Clapton and Jeff Beck, among others, the auction veteran said.