AppArmor change_hat slow when lots of hats are loaded

[John Johansen]

The apparmor change_hat api uses a linear scan of all hats loaded. Which causes it to slow down as the list gets longer. This should be transitioned to a better data structure to make the match faster.

[John Johansen (jjohansen)]

One potential improvement is when shared policy load finally gets implemented. All hats in a shared group could be scanned simitaneously using the dfa, resulting in a linear to input (hat name) cost lookup for the group. This would result in each group having a linear lookup cost.