Smack: don't show empty rules when /smack/load or /smack/load2 is read

This patch removes empty rules (i.e. with access set to '-') from the
rule list presented to user space.

Smack by design never removes labels nor rules from its lists. Access
for a rule may be set to '-' to effectively disable it. Such rules would
show up in the listing generated when /smack/load or /smack/load2 is
read. This may cause clutter if many rules were disabled.

As a rule with access set to '-' is equivalent to no rule at all, they
may be safely hidden from the listing.

Targeted for git://git.gitorious.org/smack-next/kernel.git



Signed-off-by: Rafal Krypa <r.krypa@samsung.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>