bcache: Change refill_dirty() to always scan entire disk if necessary

bcache: Change refill_dirty() to always scan entire disk if necessary

commit 627ccd20b4ad3ba836472468208e2ac4dfadbf03 upstream.

Previously, it would only scan the entire disk if it was starting from
the very start of the disk - i.e. if the previous scan got to the end.

This was broken by refill_full_stripes(), which updates last_scanned so
that refill_dirty was never triggering the searched_from_start path.

But if we change refill_dirty() to always scan the entire disk if
necessary, regardless of what last_scanned was, the code gets cleaner
and we fix that bug too.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>