ktr - add KTR_COND_LOG

ktr - add KTR_COND_LOG

 * KTR_COND_LOG provides conditional logging; the second parameter passed
   in is a condition which determines whether the entry is logged or
   not.

 * It provides a neater way to log conditionally than having to wrap a
   KTR in an if, which may then end up being empty if KTR is disbled.