SUNRPC: Add xdr_pad_size() helper

Introduce a helper function to compute the XDR pad size of a
variable-length XDR object.

Clean up: Replace open-coded calculation of XDR pad sizes.
I'm sure I haven't found every instance of this calculation.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/xdr.h

@@ -300,6 +300,21 @@ xdr_align_size(size_t n)
 	return (n + mask) & ~mask;
 }
 
+/**
+ * xdr_pad_size - Calculate size of an object's pad
+ * @n: Size of an object being XDR encoded (in bytes)
+ *
+ * This implementation avoids the need for conditional
+ * branches or modulo division.
+ *
+ * Return value:
+ *   Size (in bytes) of the needed XDR pad
+ */
+static inline size_t xdr_pad_size(size_t n)
+{
+	return xdr_align_size(n) - n;
+}
+
 /**
  * xdr_stream_encode_u32 - Encode a 32-bit integer
  * @xdr: pointer to xdr_stream