Currently __bdevname walks the obsolete list of block majors to
find a name for the given dev_t and falls back to unknown-block(%u,%u)
if that's not possible.  Replace this with an attempted get_gendisk() +
disk_name.  This means __bdevname can't be called from irq context
anymore, but as all old irq context callers are using bdevname() now
that fine (and I've added a big comment).