Matt Ahrens' review comments.

Porting Notes:
Man page changes dropped for the moment.  This can be reconsiled
when the final version is merged to OpenZFS.  They are accurate
now, only worded a little differently.

Requires-builders: none

module/zfs/metaslab.c

@@ -4520,6 +4520,8 @@ static void
 metaslab_trim_add(void *arg, uint64_t offset, uint64_t size)
 {
 	metaslab_t *msp = arg;
+
+	ASSERT(MUTEX_HELD(&msp->ms_lock));
 	ASSERT(msp->ms_cur_ts != NULL);
 	range_tree_add(msp->ms_cur_ts->ts_tree, offset, size);
 	if (msp->ms_prev_ts != NULL) {
@@ -4826,6 +4828,7 @@ static boolean_t metaslab_check_trim_conflict(metaslab_t *msp,
 	uint64_t new_offset;
 
 	ASSERT3U(*offset + size, <=, limit);
+	ASSERT(MUTEX_HELD(&msp->ms_lock));
 
 	if (msp->ms_trimming_ts == NULL)
 		/* no trim conflict, original offset is OK */

module/zfs/range_tree.c

@@ -521,6 +521,8 @@ uint64_t
 range_tree_find_gap(range_tree_t *rt, uint64_t start, uint64_t size)
 {
 	range_seg_t *rs;
+
+	ASSERT(MUTEX_HELD(rt->rt_lock));
 	while ((rs = range_tree_find_impl(rt, start, size)) != NULL)
 		start = rs->rs_end;
 	return (start);