[SCSI] sr: fix corrupt CD data after media change and delay

Reported-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>

If you delay 30s or more before mounting a CD after inserting it then
the kernel has the wrong value for the CD size.

http://marc.info/?t=121276133000001

The problem is in sr_test_unit_ready(): the function eats unit
attentions without adjusting the sdev->changed status.  This means
that when the CD signals changed media via unit attention, we can
ignore it.  Fix by making sr_test_unit_ready() adjust the changed
status.

Tested-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>