Make metadata allocation eager and leave data allocation lazy

This commit builds on the previous thin provisioning rework. The
simplified, eager metadata allocation allows us to decrease
fragmentation introduced into the previous change and the lazy data
allocation allows us to provide users with the same flexibility around
changing requirements that we previously provided.

src/engine/strat_engine/pool.rs

@@ -163,7 +163,7 @@ impl StratPool {
 
         let thinpool = ThinPool::new(
             pool_uuid,
-            match ThinPoolSizeParams::new(backstore.available_in_backstore()) {
+            match ThinPoolSizeParams::new(backstore.datatier_usable_size()) {
                 Ok(ref params) => params,
                 Err(e) => {
                     let _ = backstore.destroy();
@@ -406,7 +406,7 @@ impl StratPool {
                 .map(|(_, _, fs)| fs.thindev_size())
                 .sum::<Sectors>()
                 + increase
-                > self.thin_pool.min_datadev_size(&self.backstore)?
+                > self.thin_pool.total_fs_limit(&self.backstore)
         {
             Err(StratisError::Msg(format!(
                 "Overprovisioning is disabled on this pool and increasing total filesystems size by {} would result in overprovisioning",
@@ -1237,6 +1237,7 @@ mod tests {
             .unwrap()
             .pop()
             .unwrap();
+        udev_settle().unwrap();
         assert!(pool
             .set_overprov_mode(&Name::new(pool_name.to_string()), false)
             .is_err());