Use current_kernel_time() in the time compatibility wrappers

Use current_kernel_time() in the time compatibility wrappers

Since the Linux kernel's utimens family of functions uses
current_kernel_time(), we need to do the same in the context of ZFS
or else there can be discrepencies in timestamps (they go backward)
if userland code does:

	fd = creat(FNAME, 0600);
	(void) futimens(fd, NULL);

The getnstimeofday() function generally returns a slightly lower time
value.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#3006