apparmor: fix refcount leak when updating the sk_ctx

apparmor: fix refcount leak when updating the sk_ctx

Currently update_sk_ctx() transfers the plabel reference, unfortunately
it is also unconditionally put in the caller. Ideally we would make
the caller conditionally put the reference based on whether it was
transferred but for now just fix the bug by getting a reference.

Fixes: 88fec3526e841 ("apparmor: make sure unix socket labeling is correctly updated.")
Signed-off-by: John Johansen <john.johansen@canonical.com>