selinux: replace BUG_ONs with WARN_ONs in avc.c

These checks are only guarding against programming errors that could
silently grant too many permissions. These cases are better handled with
WARN_ON(), since it doesn't really help much to crash the machine in
this case.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>