Resolve deadlock between epoch(9) and various network interface
SX-locks, during if_purgeaddrs(), by not allowing to hold the epoch
read lock over typical network IOCTL code paths. This is a regression
issue after r334305.

Reviewed by: ae (network)
Differential revision: https://reviews.freebsd.org/D17647
MFC after: 1 week
Sponsored by: Mellanox Technologies