dm cache metadata: fix metadata2 format's blocks_are_clean_separate_dirty

The dm_bitset_cursor_begin() call was using the incorrect nr_entries.
Also, the last dm_bitset_cursor_next() must be avoided if we're at the
end of the cursor.

Fixes: 7f1b2159 ("dm cache metadata: use cursor api in blocks_are_clean_separate_dirty()")
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>