dm crypt: Fix data corruption with dm-crypt over RAID5

Fix corruption issue with dm-crypt on top of software raid5. Cancelled
readahead bio's that report no error, just have BIO_UPTODATE cleared
were reported as successful reads to the higher layers (and leaving
random content in the buffer cache). Already fixed in 2.6.19.

Signed-off-by: Christophe Saout <christophe@saout.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>