Fix unlocked access to ifnet address list

[rstone]

Summary

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

Test Plan

Added and removed IPs in a loop on an ifnet while traffic was actively
running on it.

[bz]

If I am not mistaken the function calls are for modules (especially 3rd party) while we should use the macros in the kernel (unless I misremember something here)?

[rstone]

Use macros for code compiled in kernel

[rstone]

so apparently this is called on nearly every packet that passes through ip_output() (according to a comparison of dtrace and netstat output). Adding an additional lock/unlock to the transmit path concerns me, but this is obviously the right fix. Does in_broadcast() really need to be called so much?