apparmor: fix incorrect type assignment when freeing proxies

sparse reports

poisoning the proxy->label before freeing the struct is resulting in
a sparse build warning.
../security/apparmor/label.c:52:30: warning: incorrect type in assignment (different address spaces)
../security/apparmor/label.c:52:30:    expected struct aa_label [noderef] <asn:4>*label
../security/apparmor/label.c:52:30:    got struct aa_label *<noident>

fix with RCU_INIT_POINTER as this is one of those cases where
rcu_assign_pointer() is not needed.

Signed-off-by: John Johansen <john.johansen@canonical.com>