md/raid5: clear R5_NeedReplace when no longer needed.

This flag is currently never cleared, which can in rare cases
trigger a warn-on if it is still set but the block isn't
InSync.

So clear it when it isn't need, which includes if the replacement
device has failed.

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

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