sr: Switch the sector size back to 2048 if sr_read_sector() changed it.

sr_read_sector() is hardly used since v2.3.16. Its only purpose is to
check if it is a XA medium via sr_is_xa(). This check is only enabled if
the module parameter `xa_test' is enabled.

Change the sector size back to 2048 if it was changed. With this change,
there is no lazy sector size changing left.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>