netfilter: nf_tables: rename struct nft_set_estimate class field

Use lookup as field name instead, to prepare the introduction of the
memory class in a follow up patch.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -244,11 +244,11 @@ enum nft_set_class {
  *				  characteristics
  *
  *	@size: required memory
- *	@class: lookup performance class
+ *	@lookup: lookup performance class
  */
 struct nft_set_estimate {
 	unsigned int		size;
-	enum nft_set_class	class;
+	enum nft_set_class	lookup;
 };
 
 struct nft_set_ext;