btrfs: clear defragmented inodes using postorder in btrfs_cleanup_defrag_inodes()

btrfs_cleanup_defrag_inodes() is not called frequently, only in remount
or unmount, but the way it frees the inodes in fs_info->defrag_inodes
is inefficient. Each time it needs to locate first node, remove it,
potentially rebalance tree until it's done. This allows to do a
conditional reschedule.

For cleanups the rbtree_postorder_for_each_entry_safe() iterator is
convenient but we can't reschedule and restart iteration because some of
the tree nodes would be already freed.

The cleanup operation is kmem_cache_free() which will likely take the
fast path for most objects so rescheduling should not be necessary.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>