[PATCH] pktcdvd: Only return -EROFS when appropriate

When attempting to open the device for writing, only return -EROFS if the disc
appears to be readable but not writable.

Signed-off-by: Peter Osterlund <petero2@telia.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>