common: fix _require_btime for lazy filesystems

common: fix _require_btime for lazy filesystems

Filesystems are not required to try to fill the statx btime field
unless the caller actually sets STATX_BTIME.  They're allowed to
volunteer that information "if it's cheap", but XFS doesn't
volunteer and there may be filesystems that support btime but not
cheaply.

Either way, we want to test btime on any filesystem that supports
it, cheaply or otherwise, so set STATX_BTIME when we're trying to
detect support for it.

[Eryu: fix _require_scratch_btime too]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>