gfs2: call ktime_get_coarse_real_ts64() directly

current_kernel_time64() is now just a deprecated wrapper around
ktime_get_coarse_real_ts64(), so let's just call that directly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>