common/rc: force mkfs for xfs in _mkfs_dev

common/rc: force mkfs for xfs in _mkfs_dev

In generic/459, if the DISCARD of the thin device somehow fails with
this message:

device-mapper: thin: Data device (dm-1) discard unsupported: Disabling discard passdown.

Then we can end up with arbitrary gunk in the thin device.  This
causes mkfs to fail because it's afraid to format the device.  Don't
be afraid, just zap it.  FWIW mkfs.xfs thinks that the thinp device
has an xfs external log because sometimes the thinp device just
happen to be backed by the log of the previous test's scratch fs.

Fix this by making the _mkfs_dev helper always format the device,
per Eryu Guan's suggestion.

[eguan: fix subject to reflect the fix is in common/rc]

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