fs: add timespec64_truncate()

As vfs moves to using struct timespec64 to represent times,
update the argument to timespec_truncate() to use
struct timespec64. Also change the name of the function.
The rest of the implementation logic is the same.

Move this to fs/inode.c instead of kernel/time/time.c as all the
users of this api are filesystems.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: <viro@zeniv.linux.org.uk>