Allow metaslab to be unloaded even when not freed from

On large systems, the memory used by loaded metaslabs can become
a concern. While range trees are a fairly efficient data structure, 
on heavily fragmented pools they can still consume a significant 
amount of memory. This problem is amplified when we fail to unload 
metaslabs that we aren't using. Currently, we only unload a metaslab 
during metaslab_sync_done; in order for that function to be called 
on a given metaslab in a given txg, we have to have dirtied that 
metaslab in that txg. If the dirtying was the result of an allocation, 
we wouldn't be unloading it (since it wouldn't be 8 txgs since it 
was selected), so in effect we only unload a metaslab during txgs 
where it's being freed from.

We move the unload logic from sync_done to a new function, and 
call that function on all metaslabs in a given vdev during 
vdev_sync_done().

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #8837

include/sys/metaslab.h

@@ -50,6 +50,7 @@ int metaslab_init(metaslab_group_t *, uint64_t, uint64_t, uint64_t,
 void metaslab_fini(metaslab_t *);
 
 int metaslab_load(metaslab_t *);
+void metaslab_potentially_unload(metaslab_t *, uint64_t);
 void metaslab_unload(metaslab_t *);
 
 uint64_t metaslab_allocated_space(metaslab_t *);