kernel - RB_SCAN() requires a short-term spinlock

kernel - RB_SCAN() requires a short-term spinlock

* RB_SCAN() links and unlinks an info structure.  If called with a shared
  lock the linking and unlinking operations requires a very short-term spin
  lock to avoid clobbering each other.

* Note that RB_REMOVE() scans the inprog list but this function can only
  be safely called with the RB tree held exclusively anyway, so there's
  no need to spinlock the info list.

* Add kern/subr_rbtree.c glue functions for acquiring and releasing the
  spinlock, so sys/tree.h only needs to include sys/spinlock.h and not
  also sys/spinlock2.h

Reported-by: sephe