RAID: Fix broken allocation policies for parity RAID types

RAID: Fix broken allocation policies for parity RAID types

A previous commit (b6bfddcd0a830d0c9312bc3ab906cb3d1b7a6dd9) which
was designed to prevent segfaults during lvextend when trying to
extend striped logical volumes forgot to include calculations for
RAID4/5/6 parity devices.  This was causing the 'contiguous' and
'cling_by_tags' allocation policies to fail for RAID 4/5/6.

The solution is to remember that while we can compare
	ah->area_count == prev_lvseg->area_count
for non-RAID, we should compare
	(ah->area_count + ah->parity_count) == prev_lvseg->area_count
for a general solution.