1/ make sure raid5 doesn't try to handle multiple overlaping
    requests at the same time as this would confuse things badly.
    Currently it justs BUGs if this is attempted.
 2/ Fix a possible data-loss-on-write problem.  If two or
    more bio's that write to the same page are processed at the
    same time, only the first was actually commited to storage.
 3/ Fix a use-after-free bug.  raid5 keeps the bio's it is given
    in linked lists when more than one bio touch a single page.
    In some cases the tail of this list can be freed, and
    the current test for 'are we at the end' isn't reliable.
    This patch strengths the test to make it reliable.