netfilter: ipset: Use is_zero_ether_addr instead of static and memcmp

To make the test a bit clearer and to reduce object size a little.

Miscellanea:

o remove now unnecessary static const array

$ size ip_set_hash_mac.o*
   text	   data	    bss	    dec	    hex	filename
  22822	   4619	     64	  27505	   6b71	ip_set_hash_mac.o.allyesconfig.new
  22932	   4683	     64	  27679	   6c1f	ip_set_hash_mac.o.allyesconfig.old
  10443	   1040	      0	  11483	   2cdb	ip_set_hash_mac.o.defconfig.new
  10507	   1040	      0	  11547	   2d1b	ip_set_hash_mac.o.defconfig.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>