dm snapshot: abstract minimum_chunk_size fn

The origin needs to find minimum chunksize of all snapshots.  This logic is
moved to a separate function because it will be used at another place in
the snapshot merge patches.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>