jffs2: use 64-bit intermediate timestamps

The VFS now uses timespec64 timestamps consistently, but jffs2 still
converts them to 32-bit numbers on the storage medium. As the helper
functions for the conversion (get_seconds() and timespec_to_timespec64())
are now deprecated, let's change them over to the more modern
replacements.

This keeps the traditional interpretation of those values, where
the on-disk 32-bit numbers are taken to be negative numbers, i.e.
dates before 1970, on 32-bit machines, but future numbers past 2038
on 64-bit machines.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>