fs: fat: Eliminate timespec64_trunc() usage

timespec64_trunc() is being deleted.

timestamp_truncate() is the replacement api for
timespec64_trunc. timestamp_truncate() additionally clamps
timestamps to make sure the timestamps lie within the
permitted range for the filesystem.

But, fat always truncates the times locally after it obtains
the timestamps from current_time().
Implement a local version here along the lines of existing
truncate functions.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: hirofumi@mail.parknet.co.jp
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>