model: update mo_may_allow restrictions

For future values, we can enforce the following rule:

  If X --hb-> Y --mo-> Z, then X should not read from Z.

This a change from previous behavior, where we used 'sb' instead of
'hb'.

Tested with linuxrwlocks example:

  ./run.sh test/linuxrwlocks.o -f 4 -m 1

No difference in number of executions (feasible or infeasible); HASH
values were exactly the same.