netlabel: Fix some sparse warnings

Fix a few sparse warnings.  One dealt with a RCU lock being held on error,
another dealt with an improper type caused by a signed/unsigned mixup while
the rest appeared to be caused by using rcu_dereference() in a
list_for_each_entry_rcu() call.  The latter probably isn't a big deal, but
I derive a certain pleasure from knowing that the net/netlabel is nice and
clean.

Thanks to James Morris for pointing out the issues and demonstrating how
to run sparse.

Signed-off-by: Paul Moore <paul.moore@hp.com>