Replace current_kernel_time() with getnstimeofday()

current_kernel_time() is used by the SLAB, 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>