btrfs: Add datacow mount option.

Add datacow mount option to enable copy-on-write with
remount option.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

Documentation/filesystems/btrfs.txt

@@ -154,9 +154,10 @@ Options with (*) are default options and will not show in the mount options.
 	(non-battery-backed) write-back cache, nobarrier option will lead to
 	filesystem corruption on a system crash or power loss.
 
+  datacow(*)
   nodatacow
-	Disable data copy-on-write for newly created files.  Implies nodatasum,
-	and disables all compression.
+	Enable/disable data copy-on-write for newly created files.
+	Nodatacow implies nodatasum, and disables all compression.
 
   nodatasum
 	Disable data checksumming for newly created files.