repart: make sure to grow partition table after growing backing loopback file

This fixes the --size= switch, i.e. where we grow a disk image: after
growing it we need to expand the partition table so that its idea of the
the medium size matches the new reality. Otherwise our disk size
calculations in the subsequent steps might still use the original
ungrown size.

(This used to work, I guess this was borked when libfdisk learnt the
concept of "minimized" partition tables)

(cherry picked from commit f9b3afae)