Further improvements to LOCK_PROFILING:

• Fix missing initialization in kern_rwlock.c causing bogus times to be collected
• Move updates to the lock hash to after the lock is released for spin mutexes, sleep mutexes, and sx locks
• Add new kernel build option LOCK_PROFILE_FAST - only update lock profiling statistics when an acquisition is contended. This reduces the overhead of LOCK_PROFILING to increasing system time by 20%-25% which on "make -j8 kernel-toolchain" on a dual woodcrest is unmeasurable in terms of wall-clock time. Contrast this to enabling lock profiling without LOCK_PROFILE_FAST and I see a 5x-6x slowdown in wall-clock time.