Fix unlocked access to ifnet address list

in_broadcast() was iterating over the ifnet address list without
first taking an IF_ADDR_RLOCK. This could cause a panic if a
concurrent operation modified the list.

Reviewed by: bz
MFC after: 2 months
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7227