netfilter:ipset: Fix memory allocation for bitmap:port

At the restructuring of the bitmap types creation in ipset, for the
bitmap:port type wrong (too large) memory allocation was copied
(netfilter bugzilla id #859).

Reported-by: Quentin Armitage <quentin@armitage.org.uk>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>