btrfs: make tree iteration in extent_io_tree_release() more efficient

Currently extent_io_tree_release() is a loop that keeps getting the first
node in the io tree, using rb_first() which is a loop that gets to the
leftmost node of the rbtree, and then for each node it calls rb_erase(),
which often requires rebalancing the rbtree.

We can make this more efficient by using
rbtree_postorder_for_each_entry_safe() to free each node without having
to delete it from the rbtree and without looping to get the first node.

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