btrfs: use the new bit BTRFS_FS_TREE_MOD_LOG_USERS at btrfs_free_tree_block()

Instead of exposing implementation details of the tree mod log to check
if there are active tree mod log users at btrfs_free_tree_block(), use
the new bit BTRFS_FS_TREE_MOD_LOG_USERS for fs_info->flags instead. This
way extent-tree.c does not need to known about any of the internals of
the tree mod log and avoids taking a lock unnecessarily as well.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -3342,11 +3342,9 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
 		 * find a node pointing to this leaf and record operations that
 		 * point to this leaf.
 		 */
-		if (btrfs_header_level(buf) == 0) {
-			read_lock(&fs_info->tree_mod_log_lock);
-			must_pin = !list_empty(&fs_info->tree_mod_seq_list);
-			read_unlock(&fs_info->tree_mod_log_lock);
-		}
+		if (btrfs_header_level(buf) == 0 &&
+		    test_bit(BTRFS_FS_TREE_MOD_LOG_USERS, &fs_info->flags))
+			must_pin = true;
 
 		if (must_pin || btrfs_is_zoned(fs_info)) {
 			btrfs_redirty_list_add(trans->transaction, buf);