crimson/os/seastore/async_cleaner: fix incorrect get_num_rolls()

crimson/os/seastore/async_cleaner: fix incorrect get_num_rolls()

The number of journal segments should not be based on the committed
journal_head. Otherwise, if a new journal segment is just opened and the
committed journal_head hasn't been updated, the result will be wrong.

This causes ceph_assert(get_segments_reclaimable() == 0) in
SegmentCleaner::get_next_reclaim_segment().

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>