Make rn_match() and rn_lookup() safe to be used in parrallel, when

Make rn_match() and rn_lookup() safe to be used in parrallel, when
different trees are manipulated:

- Stop writting to global variables
- Use a buffer on the stack
- Anotate read/only arrays as such

While here introduce a SALEN() macro and assert that the KERNEL_LOCK()
is held when a tree is modified.

ok bluhm@