Make conversion from a synced 'mirror' to 'raid1' not cause a full resync.

Make conversion from a synced 'mirror' to 'raid1' not cause a full resync.

It was not possible to pass down the DM_[FORCE|NO]SYNC flags to
'dm_tree_node_add_raid_target'.  This meant that converting to 'raid1' from
'mirror' would cause a full resync.  (It also meant that '--nosync' was
ineffective when creating a 'raid1' LV.)

I've taken the 'reserved' parameter in 'dm_tree_node_add_raid_target' and
used it for the "flags" parameter.  Now it is possible to pass the sync
flags and any other flags that may come up.