UPSTREAM: selinux: fix non-MLS handling in mls_context_to_sid()

Commit 95ffe194204a ("selinux: refactor mls_context_to_sid() and make
it stricter") inadvertently changed how we handle labels that did not
contain MLS information.  This patch restores the proper behavior in
mls_context_to_sid() and adds a comment explaining the proper
behavior to help ensure this doesn't happen again.

Fixes: 95ffe194204a ("selinux: refactor mls_context_to_sid() and make it stricter")
Reported-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>
(cherry picked from commit 877181a8d9dc663f7a73f77f50af714d7888ec3b)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I503260e72b6b05b1d4f2be7bc8b04e4538ca5c5f