lvconvert: fix RAID SubLV --splitmirror regression

lvconvert: fix RAID SubLV --splitmirror regression

Commit 088b3d036a73a7d947f6e9b12e8fad8a9f40d97f allowed repair on cache origin RAID LVs
and restricted lvconvert actions on RAID SubLVs to change number of mirrors, repair,
replace and type changes in order to avoid unsuitable coversions on them.

This introduced a regression prohibiting --splitmirrors on any RAID SubLVs
(e.g. of cache or thin LVs; lvconvert-{cache,thin}-raid.sh tests failing).

Fix allows split mirrors again.

Fix some indenting whilst on it.