mac80211: use offsetofend()

This was created using the following spatch:
    @find@
    type S;
    expression M, M2;
    position p;
    @@
    offsetof(S, M) + sizeof(M2)@p

    @script:python@
    m << find.M;
    m2 << find.M2;
    @@
    if not m2.endswith('-> ' + m):
            cocci.include_match(False)

    @change@
    type find.S;
    expression find.M, find.M2;
    position find.p;
    @@
    -offsetof(S, M) + sizeof(M2)@p
    +offsetofend(S, M)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>