macsec: fix NULL dereference in macsec_upd_offload()

macsec_upd_offload() gets the value of MACSEC_OFFLOAD_ATTR_TYPE
without checking its presence in the request message, and this causes
a NULL dereference. Fix it rejecting any configuration that does not
include this attribute.

Reported-and-tested-by:  <syzbot+7022ab7c383875c17eff@syzkaller.appspotmail.com>
Fixes: dcb780fb ("net: macsec: add nla support for changing the offloading selection")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>