if_detach() does if_remove(ifp); NET_LOCK(); rti_delete(). New

if_detach() does if_remove(ifp); NET_LOCK(); rti_delete().  New
igmp groups may join while sleeping in interface destruction.  In
this case if_get() in igmp_joingroup() fails and rti_fill() is not
called.  Then inm->inm_rti may be NULL.  This is the condition when
syzkaller crashes in igmp_leavegroup().
Pass the ifp the current CPU is already holding down to igmp_joingroup()
and igmp_leavegroup() to avoid half constructed igmp groups.  Calling
if_get() in caller and callee makes no sense anyway.
Reported-by: syzbot+146823a676b7bea83649@syzkaller.appspotmail.com
OK denis@