pktcdvd: sort set_blocksize() calls out

1) it doesn't make any sense to have ->open() call set_blocksize() on the
device being opened - the caller will override that anyway.

2) setting block size on underlying device, OTOH, ought to be done when
we are opening it exclusive - i.e. as part of pkt_open_dev().  Having
it done at setup time doesn't guarantee us anything about the state
at the time we start talking to it.  Worse, if you happen to have
the underlying device containing e.g. ext2 with 4Kb blocks that
is currently mounted r/o, that set_blocksize() will confuse the hell
out of filesystem.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>