Use cached feature info in spa_add_feature_stats()

Avoid issuing I/O to the pool when retrieving feature flags information.
Trying to read the ZAPs from disk means that zpool clear would hang if
the pool is suspended and recovery would require a reboot. To keep the
feature stats resident in memory, we hang a cached nvlist off of the
spa.  It is built up from disk the first time spa_add_feature_stats() is
called, and refreshed thereafter using the cached feature reference
counts. spa_add_feature_stats() gets called at pool import time so we
can be sure the cached nvlist will be available if the pool is later
suspended.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3082

module/zfs/spa_misc.c

@@ -643,6 +643,7 @@ spa_remove(spa_t *spa)
 
 	nvlist_free(spa->spa_label_features);
 	nvlist_free(spa->spa_load_info);
+	nvlist_free(spa->spa_feat_stats);
 	spa_config_set(spa, NULL);
 
 	refcount_destroy(&spa->spa_refcount);