netfilter: ipset: kernel: uapi: fix MARKMASK attr ABI breakage

commit 2dfb973c0dcc6d2211 (add markmask for hash:ip,mark data type)
inserted IPSET_ATTR_MARKMASK in-between other enum values, i.e.
changing values of all further attributes.  This causes 'ipset list'
segfault on existing kernels since ipset no longer finds
IPSET_ATTR_MEMSIZE (it has a different value on kernel side).

Jozsef points out it should be moved below IPSET_ATTR_MARK which
works since there is some extra reserved space after that value.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>