md/raid10: fix conversion from RAID0 to RAID10

A RAID0 array (like a LINEAR array) does not have a concept
of 'size' being the amount of each device that is in use.
Rather, as much of each device as is available is used.
So the 'size' is set to 0 and ignored.

RAID10 does have this concept and needs it to be set correctly.
So when we convert RAID0 to RAID10 we must determine the
'size' (that being the size of the first 'strip_zone' in the
RAID0), and set it correctly.

Reported-and-tested-by: Xiao Ni <xni@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>