md/raid10: remove unnecessary smp_mb() from end_sync_write

Recent commit 4ca40c2c (md/raid10: Allow replacement device ...)
added an smp_mb in end_sync_write.
This was to close a possible race with raid10_remove_disk.
However there is no such race as it is never attempted to remove a
disk while resync (or recovery) is happening.
so the smp_mb is just noise.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -1682,10 +1682,8 @@ static void end_sync_write(struct bio *bio, int error)
 	d = find_bio_disk(conf, r10_bio, bio, &slot, &repl);
 	if (repl)
 		rdev = conf->mirrors[d].replacement;
-	if (!rdev) {
-		smp_mb();
+	else
 		rdev = conf->mirrors[d].rdev;
-	}
 
 	if (!uptodate) {
 		if (repl)