md/raid5: rearrange a test in fetch_block6.

Next patch will unite fetch_block5 and fetch_block6.
First I want to make the differences a little more clear.

For RAID6 if we are writing at all and there is a failed device, then
we need to load or compute every block so we can do a
reconstruct-write.
This case isn't needed for RAID5 - we will do a read-modify-write in
that case.
So make that test a separate test in fetch_block6 rather than merged
with two other tests.

Make a similar change in fetch_block5 so the one bit that is not
needed for RAID6 is clearly separate.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Namhyung Kim <namhyung@gmail.com>