md/raid5: need_this_block: start simplifying the last two conditions.

Both the last two cases are only relevant if something has failed and
something needs to be written (but not over-written), and if it is OK
to pre-read blocks at this point.  So factor out those tests and
explain them.

Signed-off-by: NeilBrown <neilb@suse.de>

diff --cc drivers/md/raid5.c
Simple merge