apparmor: fix loop detection used in conflicting attachment resolution

Conflicting attachment resolution is based on the number of states
traversed to reach an accepting state in the attachment DFA, accounting
for DFA loops traversed during the matching process. However, the loop
counting logic had multiple bugs:

- The inc_wb_pos macro increments both position and length, but length
is supposed to saturate upon hitting buffer capacity, instead of
wrapping around.
- If no revisited state is found when traversing the history, is_loop
would still return true, as if there was a loop found the length of
the history buffer, instead of returning false and signalling that
no loop was found. As a result, the adjustment step of
aa_dfa_leftmatch would sometimes produce negative counts with loop-
free DFAs that traversed enough states.
- The iteration in the is_loop for loop is supposed to stop before
i = wb->len, so the conditional should be < instead of <=.

This patch fixes the above bugs as well as the following nits:
- The count and size fields in struct match_workbuf were not used,
so they can be removed.
- The history buffer in match_workbuf semantically stores aa_state_t
and not unsigned ints, even if aa_state_t is currently unsigned int.
- The local variables in is_loop are counters, and thus should be
unsigned ints instead of aa_state_t's.

Fixes: 21f606610502 ("apparmor: improve overlapping domain attachment resolution")

Signed-off-by: Ryan Lee <ryan.lee@canonical.com>
Co-developed-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>