btrfs: rename BTRFS_ROOT_REF_COWS to BTRFS_ROOT_SHAREABLE

The name BTRFS_ROOT_REF_COWS is not very clear about the meaning.

In fact, that bit can only be set to those trees:

- Subvolume roots
- Data reloc root
- Reloc roots for above roots

All other trees won't get this bit set.  So just by the result, it is
obvious that, roots with this bit set can have tree blocks shared with
other trees.  Either shared by snapshots, or by reloc roots (an special
snapshot created by relocation).

This patch will rename BTRFS_ROOT_REF_COWS to BTRFS_ROOT_SHAREABLE to
make it easier to understand, and update all comment mentioning
"reference counted" to follow the rename.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/tree-defrag.c b/fs/btrfs/tree-defrag.c
index 5f9e2dd..16c3a6d 100644
--- a/fs/btrfs/tree-defrag.c
+++ b/fs/btrfs/tree-defrag.c
@@ -35,7 +35,7 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 		goto out;
 	}
 
-	if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state))
+	if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
 		goto out;
 
 	path = btrfs_alloc_path();