apparmor: fix overlapping attachment computation

When finding the profile via patterned attachments, the longest left
match is being set to the static compile time value and not using the
runtime computed value.

Fix this by setting the candidate value to the greater of the
precomputed value or runtime computed value.

Fixes: 21f60661 ("apparmor: improve overlapping domain attachment resolution")
Signed-off-by: John Johansen <john.johansen@canonical.com>