fs: ubifs: Eliminate timespec64_trunc() usage

DEFAULT_TIME_GRAN is seconds granularity. We can
just drop the nsec while creating the default root node.
Delete the unneeded call to timespec64_trunc().

Also update the ktime_get_* api to match the one used in
current_time(). This allows for the timestamps to be updated
by using the same ktime_get_* api always.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: richard@nod.at
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>