Some minor cleanups:

Some minor cleanups:
- In rt_check() remove the senderr() macro and the "bad" label. They
  used to simplify code, but now aren't.
- Remove extra RT_LOCK_ASSERT() in rt_setgate(). The RT_REMREF macro
  does this.
- In rtfree() convert panics to KASSERTs.
- Strict the routing API: rtfree() should be called only in a case
  when we are completely sure we've got the last reference on the
  rtentry. In all other cases RTFREE_LOCKED() macro should be used.
  If the reference isn't the last one spit out a warning printf.
  Correct the only(?) case for this in rt_check().
- Fix typos in comments.