From: Neil Brown <neilb@cse.unsw.edu.au>

With the current code in bio_add_page, if fail_segments is ever set, it
stays set, so bio_add_page will eventually fail having recounted the
segmentation once.

I don't think this is intended.  This patch changes the code to allow
success if the recounting the segments helps.

(Jens has acked this)