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

include/sys/spa_impl.h

@@ -236,6 +236,7 @@ struct spa {
 	uint64_t	spa_feat_for_read_obj;	/* required to read from pool */
 	uint64_t	spa_feat_desc_obj;	/* Feature descriptions */
 	uint64_t	spa_feat_enabled_txg_obj; /* Feature enabled txg */
+	nvlist_t	*spa_feat_stats;	/* Cache of enabled features */
 	/* cache feature refcounts */
 	uint64_t	spa_feat_refcount_cache[SPA_FEATURES];
 	taskqid_t	spa_deadman_tqid;	/* Task id */