apparmor: reduce rcu_read_lock scope for aa_file_perm mediation

Now that the buffers allocation has changed and no longer needs
the full mediation under an rcu_read_lock, reduce the rcu_read_lock
scope to only where it is necessary.

Fixes: df323337 ("apparmor: Use a memory pool instead per-CPU caches")
Signed-off-by: John Johansen <john.johansen@canonical.com>