Canelo Alvarez Down To Box UFC's Jorge Masvidal, 'Why Not?!'

Forget Conor McGregor ... Canelo Alvarez says he's down to fight UFC star Jorge Masvidal next -- telling TMZ Sports, "Why not?!" ... Of course, in boxing only. Masvidal famously called out Canelo shortly after beating Nate Diaz for the 'BMF' belt…