dm thin metadata: introduce dm_pool_abort_metadata

Introduce dm_pool_abort_metadata to abort the current metadata
transaction.  Generally this will only be called when bad things are
happening and dm-thin is trying to roll back to a good state for
read-only mode.

It's complicated by the fact that the metadata device may have failed
completely causing the abort to be unable to read the old transaction.
In this case the metadata object is placed in a 'fail' mode and
everything fails apart from destroying it.

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>