xfs: use 40-bit quota time limits

The quota handling in xfs is based around an in-memory representation
of time_t, which overflows in year 2038 on 32-bit architectures, and an
on-disk representation of __be32, which overflows in year 2106 based
on interpreting the values as unsigned.

Extend both to allow for much longer times: the in-memory representation
should just use time64_t and the on-disk representation has to live with
the spare bits in struct xfs_disk_dquot. As there is an unused 32-bit
field, and three time limits in it, allocating 8 bits per timeout
seems appropriate.

Note: the quotactl() syscall is not affected by this, it has its
own struct fs_disk_quota that may need a similar conversion.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>