OK, here comes the long-promised switchover to ->get_sb().

New method added - ->get_sb(type, flags, dev, data).  At this point we
still keep ->read_super(), but it will go away in the end of series. 
->get_sb() is a generalization of get_sb_{bdev,nodev,single}() - as the
matter of fact, these will become common helpers used by filesystems in
their ->get_sb().