netfilter: ipset: Removed invalid IPSET_ATTR_MARKMASK validation

Markmask is an u32, hence it can't be greater then 4294967295 ( i.e.
0xffffffff ). This was causing smatch warning:
 net/netfilter/ipset/ip_set_hash_gen.h:1084 hash_ipmark_create() warn:
 impossible condition '(markmask > 4294967295) => (0-u32max > u32max)'

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>