Push kernel lock down to rt_setsource() to make `ifa' dereference safe.

Push kernel lock down to rt_setsource() to make `ifa' dereference safe.
Netlock doesn't make sense here because ifa_ifwithaddr() holds kernel
lock while performs lists walkthrough.

This was made to decrease the future diff for PF_ROUTE sockets
unlocking. This time kernel lock is still held while we perform
rt_setsource().

ok mpi@