Btrfs: remove noused bbio_ret in __btrfs_map_block in condition

bbio_ret in this condition is always !NULL because previous code
already have a check-and-skip:
4908 if (!bbio_ret)
4909     goto out;

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>

fs/btrfs/volumes.c

@@ -5161,8 +5161,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int rw,
 				BTRFS_BLOCK_GROUP_RAID6)) {
 		u64 tmp;
 
-		if (bbio_ret && ((rw & REQ_WRITE) || mirror_num > 1)
-		    && raid_map_ret) {
+		if (raid_map_ret && ((rw & REQ_WRITE) || mirror_num > 1)) {
 			int i, rot;
 
 			/* push stripe_nr back to the start of the full stripe */