lvconvert: fix interim segtype regression on raid6 conversions

lvconvert: fix interim segtype regression on raid6 conversions

When converting from striped/raid0/raid0_meta
to raid6 with > 2 stripes, allow possible
direct conversion (to raid6_n_6).

In case of 2 stripes, first convert to raid5_n to restripe
to at least 3 data stripes (the raid6 minimum in lvm2) in
a second conversion before finally converting to raid6_n_6.

As before, raid6_n_6 then can be converted
to any other raid6 layout.

Enhance lvconvert-raid-takeover.sh to test the
2 stripes conversions to raid6.

Resolves: rhbz1624038