btrfs: Add datasum mount option.

Add datasum mount option to enable checksum 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

@@ -159,8 +159,10 @@ Options with (*) are default options and will not show in the mount options.
 	Enable/disable data copy-on-write for newly created files.
 	Nodatacow implies nodatasum, and disables all compression.
 
+  datasum(*)
   nodatasum
-	Disable data checksumming for newly created files.
+	Enable/disable data checksumming for newly created files.
+	Datasum implies datacow.
 
   notreelog
 	Disable the tree logging used for fsync and O_SYNC writes.