ubifs: use timespec64 for inode timestamps

Both vfs and the on-disk inode structures can deal with fine-grained
timestamps now, so this is the last missing piece to make ubifs
y2038-safe on 32-bit architectures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Weinberger <richard@nod.at>