SELinux: delay initialization of audit data in selinux_inode_permission

We pay a rather large overhead initializing the common_audit_data.
Since we only need this information if we actually emit an audit
message there is little need to set it up in the hot path.  This patch
splits the functionality of avc_has_perm() into avc_has_perm_noaudit(),
avc_audit_required() and slow_avc_audit().  But we take care of setting
up to audit between required() and the actual audit call.  Thus saving
measurable time in a hot path.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Eric Paris <eparis@redhat.com>