lvconvert: fix conversion to 'mirrored' mirror log with larger regionsize

lvconvert: fix conversion to 'mirrored' mirror log with larger regionsize

merge.c:_check_lv_segment() was checking regionsize vs. mirrored LV size on
any 'mirror/raid1/raid10' segment type including type 'mirrored' mirror logs.

Avoid the check only for 'mirrored' mirror logs to allow conversion from log
type 'disk' with regionsize > mirror log SubLV size.

As we disabled support for 'mirrored' mirror logs with
commit e82303fd6abc3ae43168f8032806c7c17d181a3e which still conditionally
allows to enable it via global/support_mirrored_mirror_logs=1,
patch is mandatory for all distributions.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1712983