Fix readahead calculation problems.

Fix readahead calculation problems.

During vgreduce is failed mirror image replaced with error segment,
this segmant type has always area_count == 0.
Current code expects that there is at least one area with device,
patch fixes it by additional check (fixes segfault during vgreduce).

Also do not calculate readahead in every lv_info call, we only need
to cache PV readahead before activation calls which locks memory.