Don't lock rt_lock in range_tree_verify()

range_tree_verify() was the only range tree support function which
locked rt_lock whereas all the other functions required the lock to
be taken by the caller.  If the lock is taken in range_tree_verify(),
it's not possible to atomically verify a set of related range trees
(those which are likely protected by the same lock).

In the previous implementation, checking "related" trees would be done
as follows:

    range_tree_verify(tree1, offset, size);
    /* tree1's rt_lock is not taken here */
    range_tree_verify(tree2, offset, size);

The new implementation requires:

    mutex_enter(tree1->rt_lock);
    range_tree_verify(tree1, offset, size);
    range_tree_verify(tree2, offset, size);
    mutex_exit(tree1->rt_lock);

Currently, the only consumer of range_tree_verify() is
metaslab_check_free() which verifies a set of realted range trees in
a metaslab.  The TRIM/DISCARD code adds an additional set of checks of
the current and previous trimsets, both of which are represented as
range trees.

metaslab_check_free() has been updated to lock ms_lock once for each
vdev's metaslab and also for debugging builds to verify that the each
tree's rt_lock matches the metaslab's ms_lock to prove they're related.

module/zfs/metaslab.c

@@ -3617,12 +3617,18 @@ metaslab_check_free(spa_t *spa, const blkptr_t *bp)
 		uint64_t size = DVA_GET_ASIZE(&bp->blk_dva[i]);
 		metaslab_t *msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];
 
+		mutex_enter(&msp->ms_lock);
 		if (msp->ms_loaded) {
+			VERIFY(&msp->ms_lock == msp->ms_tree->rt_lock);
 			range_tree_verify(msp->ms_tree, offset, size);
 #ifdef	DEBUG
+			VERIFY(&msp->ms_lock ==
+			    msp->ms_cur_ts->ts_tree->rt_lock);
 			range_tree_verify(msp->ms_cur_ts->ts_tree,
 			    offset, size);
 			if (msp->ms_prev_ts != NULL) {
+				VERIFY(&msp->ms_lock ==
+				    msp->ms_prev_ts->ts_tree->rt_lock);
 				range_tree_verify(msp->ms_prev_ts->ts_tree,
 				    offset, size);
 			}
@@ -3633,6 +3639,7 @@ metaslab_check_free(spa_t *spa, const blkptr_t *bp)
 		range_tree_verify(msp->ms_freedtree, offset, size);
 		for (j = 0; j < TXG_DEFER_SIZE; j++)
 			range_tree_verify(msp->ms_defertree[j], offset, size);
+		mutex_exit(&msp->ms_lock);
 	}
 	spa_config_exit(spa, SCL_VDEV, FTAG);
 }

module/zfs/range_tree.c

@@ -338,11 +338,9 @@ range_tree_verify(range_tree_t *rt, uint64_t off, uint64_t size)
 {
 	range_seg_t *rs;
 
-	mutex_enter(rt->rt_lock);
 	rs = range_tree_find(rt, off, size);
 	if (rs != NULL)
 		panic("freeing free block; rs=%p", (void *)rs);
-	mutex_exit(rt->rt_lock);
 }
 
 boolean_t