Make BIO_UPTODATE a sticky good flag, only clearing it on error.

I pondered leaving the BIO_UPTODATE clearing up to the bi_end_io()
functions, but decided against that and left it in bio_endio() as
before.

I also added a bit of debug in bio_endio() to detect callers specifying
more than bi_size i/o complete.  And to top it all of, bio_endio()
documentation.  This is mainly to clear my back when people bitch about
using bio_endio(), I can say RTFM without worry :-)