Initial support for Smack rule caching and access checking

This series adds initial support for (a) caching Smack rules for later use, and (b) checking access permissions via Smack for requests to get/set keys.

Missing from this series is the inotify watch for /sys/fs/smackfs/load2 (and possibly /sys/fs/smackfs/load), and the hashmap-freeing logic for the smackrules hashmap.  I haven't decided how best to implement these yet.