Btrfs: add ref_count and free function for btrfs_bio

1: ref_count is simple than current RBIO_HOLD_BBIO_MAP_BIT flag
   to keep btrfs_bio's memory in raid56 recovery implement.
2: free function for bbio will make code clean and flexible, plus
   forced data type checking in compile.

Changelog v1->v2:
 Rename following by David Sterba's suggestion:
 put_btrfs_bio() -> btrfs_put_bio()
 get_btrfs_bio() -> btrfs_get_bio()
 bbio->ref_count -> bbio->refs

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/scrub.c

@@ -855,7 +855,7 @@ static inline void scrub_get_recover(struct scrub_recover *recover)
 static inline void scrub_put_recover(struct scrub_recover *recover)
 {
 	if (atomic_dec_and_test(&recover->refs)) {
-		kfree(recover->bbio);
+		btrfs_put_bbio(recover->bbio);
 		kfree(recover);
 	}
 }
@@ -1373,13 +1373,13 @@ static int scrub_setup_recheck_block(struct scrub_ctx *sctx,
 		ret = btrfs_map_sblock(fs_info, REQ_GET_READ_MIRRORS, logical,
 				       &mapped_length, &bbio, 0, 1);
 		if (ret || !bbio || mapped_length < sublen) {
-			kfree(bbio);
+			btrfs_put_bbio(bbio);
 			return -EIO;
 		}
 
 		recover = kzalloc(sizeof(struct scrub_recover), GFP_NOFS);
 		if (!recover) {
-			kfree(bbio);
+			btrfs_put_bbio(bbio);
 			return -ENOMEM;
 		}
 
@@ -2748,7 +2748,7 @@ static void scrub_parity_check_and_repair(struct scrub_parity *sparity)
 rbio_out:
 	bio_put(bio);
 bbio_out:
-	kfree(bbio);
+	btrfs_put_bbio(bbio);
 	bitmap_or(sparity->ebitmap, sparity->ebitmap, sparity->dbitmap,
 		  sparity->nsectors);
 	spin_lock(&sctx->stat_lock);
@@ -3879,14 +3879,14 @@ static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
 			      &mapped_length, &bbio, 0);
 	if (ret || !bbio || mapped_length < extent_len ||
 	    !bbio->stripes[0].dev->bdev) {
-		kfree(bbio);
+		btrfs_put_bbio(bbio);
 		return;
 	}
 
 	*extent_physical = bbio->stripes[0].physical;
 	*extent_mirror_num = bbio->mirror_num;
 	*extent_dev = bbio->stripes[0].dev;
-	kfree(bbio);
+	btrfs_put_bbio(bbio);
 }
 
 static int scrub_setup_wr_ctx(struct scrub_ctx *sctx,