dm persistent data: always unlock superblock in dm_bm_flush_and_unlock
Unlock the superblock even if initial dm_bufio_write_dirty_buffers fails.
Also, remove redundant flush calls. dm_bm_flush_and_unlock's calls to
dm_bufio_write_dirty_buffers already result in dm_bufio_issue_flush
being called.
This avoids warnings about unflushed dirty buffers from bufio.
Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>