md/raid5: avoid reading parity blocks for full-stripe write to degraded array

When performing a reconstruct write, we need to read all blocks
that are not being over-written .. except the parity (P and Q) blocks.

The code currently reads these (as they are not being over-written!)
unnecessarily.

Signed-off-by: NeilBrown <neilb@suse.de>
Fixes: ea664c82 ("md/raid5: need_this_block: tidy/fix last condition.")