in_mcat.c: change multicast not member condition
If there is no source filter entry => block if that's SSM ("exclude"
mode per RFC 3678 clause 3). If there is an entry => check its action &
block if the action is "exclude".
It would be nice if the test case in this PR were converted into an ATF
test case, but not blocking on that.
Reviewed by: imp, melifaro
Pull Request: https://github.com/freebsd/freebsd-src/pull/601