btrfs: print the block rsv type when we fail our reservation

To help with debugging, print the type of the block rsv when we fail to
use our target block rsv in btrfs_use_block_rsv.

This now produces:

 [  544.672035] BTRFS: block rsv 1 returned -28

which is still cryptic without consulting the enum in block-rsv.h but I
guess it's better than nothing.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
[ add note from Nikolay ]
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/block-rsv.c

@@ -511,7 +511,8 @@ struct btrfs_block_rsv *btrfs_use_block_rsv(struct btrfs_trans_handle *trans,
 				/*DEFAULT_RATELIMIT_BURST*/ 1);
 		if (__ratelimit(&_rs))
 			WARN(1, KERN_DEBUG
-				"BTRFS: block rsv returned %d\n", ret);
+				"BTRFS: block rsv %d returned %d\n",
+				block_rsv->type, ret);
 	}
 try_reserve:
 	ret = btrfs_reserve_metadata_bytes(root, block_rsv, blocksize,