btrfs: use list_for_each_entry_safe in free_reloc_roots

The function always works on a local copy of the reloc root list, which
cannot be modified outside of it so using list_for_each_entry is fine.
Additionally the macro handles empty lists so drop list_empty checks of
callers. No semantic changes.

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