xfs: quota: move to time64_t interfaces

As a preparation for removing the 32-bit time_t type and
all associated interfaces, change xfs to use time64_t and
ktime_get_real_seconds() for the quota housekeeping.

This avoids one difference between 32-bit and 64-bit kernels,
raising the theoretical limit for the quota grace period
to year 2106 on 32-bit instead of year 2038.

Note that common user space tools using the XFS quotactl
interface instead of the generic one still use the y2038
dates.

To fix quotas properly, both the on-disk format and user
space still need to be changed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>