quota: use time64_t internally

The quota subsystem has two formats, the old v1 format using architecture
specific time_t values on the on-disk format, while the v2 format
(introduced in Linux 2.5.16 and 2.4.22) uses fixed 64-bit little-endian.

While there is no future for the v1 format beyond y2038, the v2 format
is almost there on 32-bit architectures, as both the user interface
and the on-disk format use 64-bit timestamps, just not the time_t
inbetween.

This changes the internal representation to use time64_t, which will
end up doing the right thing everywhere for v2 format.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jan Kara <jack@suse.cz>