Revert part of the previous diff to fix a regression (another endless loop)

Revert part of the previous diff to fix a regression (another endless loop)
reported by Michael <Stapelberg at Debian> in the Linux md(4) manual.

The reason the colwidth[] array is needed is not that it stores widths
different from those in tbl->cols[].width, but that only part of the
columns participate in the comparisons, i.e. only those intersecting
at least one span the still requires width distribution.