selinux: refactor avtab_node comparisons

In four separate functions within avtab, the same comparison logic is
used. The only difference is how the result is handled or whether there
is a unique specifier value to be checked for or used.

Extracting this functionality into the avtab_node_cmp() function unifies
the comparison logic between searching and insertion and gets rid of
duplicative code so that the implementation is easier to maintain.

Signed-off-by: Jacob Satterfield <jsatterfield.linux@gmail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>