Change to use bio->__bi_remaining to count number of r1bio attached
to a bio.
See precious raid10 patch for more details.

Like the raid10.c patch, this fixes a bug as nr_queued and nr_pending
used to measure different things, but were being compared.

This patch fixes another bug in that nr_pending previously did not
could write-behind requests, so behind writes could continue while
resync was happening.  How that nr_pending counts all r1_bio,
the resync cannot commence until the behind writes have completed.
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>