xfs: change the seconds fields in xfs_bulkstat to signed

64-bit time is a signed quantity in the kernel, so the bulkstat
structure should reflect that.  Note that the structure size stays
the same and that we have not yet published userspace headers for this
new ioctl so there are no users to break.

Fixes: 7035f972 ("xfs: introduce new v5 bulkstat structure")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>