selinux: kill 'flags' argument in avc_has_perm_flags() and avc_audit()

... along with avc_has_perm_flags() itself, since now it's identical
to avc_has_perm() (as pointed out by Paul Moore)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[PM: add "selinux:" prefix to subj and tweak for length]
Signed-off-by: Paul Moore <paul@paul-moore.com>