wipefs: Add --force option to force it to erase.

wipefs: Add --force option to force it to erase.

Commit c550f728f724360f99aae0fdb45b0589d9a347e0 added O_EXCL when
opening the thing to erase.  This broke the wipefs utility when used
on anything which isn't an unmounted filesystem.  eg. If you use it on
a block device containing partitions, then it won't work because the
kernel recognizes the partitions and so thinks the device is in use.

This change adds the --force option which, when used, undoes the above
flag change.  However you still have to use --force most of the time
when erasing something that isn't a plain unmounted filesystem.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>