MFC 239440,239519:

MFC 239440,239519:
Refine the changes made in r208212 to avoid bogus failures from
if_delmulti() when clearing the configuration for a subinterface when
the parent interface is being detached.  The current code was still
triggering an assertion in if_delmulti() due to the parent interface being
partially detached.  Fix this by not calling if_delmulti() at all if the
parent interface is being detached.  Warn if if_delmulti() fails when the
parent is not being detached (but similar to 208212, still proceed with
tearing down the vlan state).

git-svn-id: svn://svn.freebsd.org/base/stable/9@241327 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f