Replace current_kernel_time() with getnstimeofday()

current_kernel_time() has been observed to cause ZFS timestamps to be
truncated at seconds. This precision is unacceptable for certain tasks,
so we switch to getnstimeofday(). This was suggested on stackoverflow to
someone who had a similar problem:

http://stackoverflow.com/questions/4627440/how-reliable-is-current-kernel-time

Signed-off-by: Richard Yao <ryao@gentoo.org>