RAID: Fix problems with creating, extending and converting large RAID LVs

RAID: Fix problems with creating, extending and converting large RAID LVs

MD's bitmaps can handle 2^21 regions at most.  The RAID code has always
used a region_size of 1024 sectors.  That means the size of a RAID LV was
limited to 1TiB.  (The user can adjust the region_size when creating a
RAID LV, which can affect the maximum size.)  Thus, creating, extending or
converting to a RAID LV greater than 1TiB would result in a failure to
load the new device-mapper table.

Again, the size of the RAID LV is not limited by how much space is allocated
for the metadata area, but by the limitations of the MD bitmap.  Therefore,
we must adjust the 'region_size' to ensure that the number of regions does
not exceed the limit.  I've added code to do this when extending a RAID LV
(which covers 'create' and 'extend' operations) and when up-converting -
specifically from linear to RAID1.