btrfs: rename root to log in walk_down_log_tree() and walk_up_log_tree()

btrfs: rename root to log in walk_down_log_tree() and walk_up_log_tree()

Everywhere we have a log root we name it as 'log' or 'log_root' except in
walk_down_log_tree() and walk_up_log_tree() where we name it as 'root',
which not only it's inconsistent, it's also confusing since we typically
use 'root' when naming variables that refer to a subvolume tree. So for
clairty and consistency rename the 'root' argument to 'log'.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>