[PATCH] md: Make sure bi_max_vecs is set properly in bio_split

Else a subsequent bio_clone might make a mess.

Signed-off-by: Neil Brown <neilb@suse.de>
Acked-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>