apparmor: fix logging of the existence test for signals

The existence test is not being properly logged as the signal mapping
maps it to the last entry in the named signal table. This is done
to help catch bugs by making the 0 mapped signal value invalid so
that we can catch the signal value not being filled in.

When fixing the off-by-one comparision logic the reporting of the
existence test was broken, because the logic behind the mapped named
table was hidden. Fix this by adding a define for the name lookup
and using it.

Cc: Stable <stable@vger.kernel.org>
Fixes: f7dc4c9a

 ("apparmor: fix off-by-one comparison on MAXMAPPED_SIG")
Signed-off-by: John Johansen <john.johansen@canonical.com>