generic/459: ensure that the lvm devices have been created

generic/459: ensure that the lvm devices have been created

Once in a /very/ long while this test fails because _mkfs_dev can't find
the LVM thinp volume after it's been created.  Since the "solution" du
jour seems to be to sprinkle udevadm settle calls everywhere, do that
here in the hopes that will fix it.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>