Replace current_kernel_time() with getnstimeofday()

current_kernel_time() is used by the SPLAT, but it is not meant for
performance measurement. We modify the SLAB to use getnstimeofday(),
which is equivalent to the gethrestime() function on Solaris.
Additionally, we update gethrestime() to invoke getnstimeofday().

Signed-off-by: Richard Yao <ryao@gentoo.org>