Fix bug introduced in 2.6.11-rc2: when we clone a BIO we need to copy over the
current index into it as well.

It corrupts data with some MD setups.

See http://bugzilla.kernel.org/show_bug.cgi?id=4946

Huuuuuuuuge thanks to Matthew Stapleton <matthew4196@gmail.com> for doggedly
chasing this one down.
Acked-by: Jens Axboe <axboe@suse.de>
Cc: <linux-raid@vger.kernel.org>
Cc: <dm-devel@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>