[lib] reset multipath field widths before printing topology

[lib] reset multipath field widths before printing topology

Doing a "show topo" after "show maps" gave different output than "show topo"
on a freshly started multipathd or than "multipath -ll". This is due to
"show map" doing a get_multipath_layout(), which result is persistent, and
affects all subsequent users of the snprint_*() functions.

This patch ensures a predictable output format by reseting the multipath
field widths in snprint_multipath_topology().