xfs: rename compat_time_t to old_time32_t

The compat_time_t type has been removed everywhere else,
as most users rely on old_time32_t for both native and
compat mode handling of 32-bit time_t.

Remove the last one in xfs.

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