Like with r185713 make sure to not leak a lock as rtalloc1(9) returns

Like with r185713 make sure to not leak a lock as rtalloc1(9) returns
a locked route. Thus we have to use RTFREE_LOCKED(9) to get it unlocked
and rtfree(9)d rather than just rtfree(9)d.

Since the PR was filed, new places with the same problem were added
with new code.  Also check that the rt is valid before freeing it
either way there.

PR:		kern/129793
Submitted by:	Dheeraj Reddy <dheeraj@ece.gatech.edu>
MFC after:	2 weeks
Committed from:	Bugathon #6