When detaching a network interface drain the workqueue freeing the inm's
because the destructor will access the if_ioctl() callback in the ifnet
pointer which is about to be freed. This prevents use-after-free.

PR: 233535
Differential Revision: https://reviews.freebsd.org/D18887
Reviewed by: bz (net)
Tested by: ae
MFC after: 1 week
Sponsored by: Mellanox Technologies