ceph: update description of some mount options

Based on code, default value of rsize/wsize is 16 MB.
Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Documentation/filesystems/ceph.txt

@@ -105,15 +105,13 @@ Mount Options
 	address its connection to the monitor originates from.
 
   wsize=X
-	Specify the maximum write size in bytes.  By default there is no
-	maximum.  Ceph will normally size writes based on the file stripe
-	size.
+	Specify the maximum write size in bytes.  Default: 16 MB.
 
   rsize=X
-	Specify the maximum read size in bytes.  Default: 64 MB.
+	Specify the maximum read size in bytes.  Default: 16 MB.
 
   rasize=X
-	Specify the maximum readahead.  Default: 8 MB.
+	Specify the maximum readahead size in bytes.  Default: 8 MB.
 
   mount_timeout=X
 	Specify the timeout value for mount (in seconds), in the case