[PATCH] (3/8) ->get_sb() switchover

[PATCH] (3/8) ->get_sb() switchover

ext2 switched to ->get_sb().  For that we had
        a) turned ext2_read_super() into ext2_fill_super() (instead of
"return NULL if failed and sb if succeeded" it's "return error value or
0").
        b) added ext2_get_sb(type, flags, dev, data) - all it does is
return get_sb_bdev(type, flags, dev, data, ext2_fill_super);
        c) replaced DECLARE_...() with explicit initializer.
That's it.