Remove kernel lock from ifa_ifwithaddr() and ifa_ifwithdstaddr().

Remove kernel lock from ifa_ifwithaddr() and ifa_ifwithdstaddr().
Netlock protects `if_list', `ifa_list' and returned `ifa' dereference,
so put netlock assertion within.

Please note, rtable_setsource() doesn't destroy data pointed by
`ar_source'. This is the `ifa_addr' data belongs to `ifa' and exclusive
netlock is required to destroy it. So the kernel lock is not required
within rt_setsource(). Take netlock by rt_setsource() caller to make
`ifa' dereference safe.

Suggestions and ok by bluhm@