Push kernel lock down to umb_rtrequest().

Push kernel lock down to umb_rtrequest().

We are going to unlock PF_ROUTE sockets. This means `if_rtrequest'
handler will be performed without kernel lock.

umb_rtrequest() calls umb_send_inet_proposal() which touches kernel lock
protected `ipv{4,6}dns' array. Also umb_rtrequest() is the only handler
which requires kernel lock to be held. So push the lock down to
umb_rtrequest() instead of grab it around `if_rtrequest' call.

This hunk was commited separately for decreases PF_ROUTE sockets
unlocking diff.

ok gerhard@ deraadt@