md/raid10: fix transcription error in calc_sectors conversion.

The old code was
		sector_div(stride, fc);
the new code was
		sector_dir(size, conf->near_copies);

'size' is right (the stride various wasn't really needed), but
'fc' means 'far_copies', and that is an important difference.

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

drivers/md/raid10.c

@@ -3189,7 +3189,7 @@ static void calc_sectors(struct r10conf *conf, sector_t size)
 	if (conf->far_offset)
 		conf->stride = 1 << conf->chunk_shift;
 	else {
-		sector_div(size, conf->near_copies);
+		sector_div(size, conf->far_copies);
 		conf->stride = size << conf->chunk_shift;
 	}
 }