Writing to RAM capable media was broken by the moving of write flag
detection from probe to drive open time.  The media would stay
terminally RO due to the set_disk_ro() call. 

This fixes the code to do the right thing - the detection stays at open
time (which it must, since it depends on the media), and we fail with
-EROFS if the media can't be written.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>