SUNRPC: Adjust synopsis of xdr_buf_subsegment()

Clean up: This enables xdr_buf_subsegment()'s callers to pass in a
const pointer to that buffer.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/xdr.h

@@ -183,7 +183,8 @@ xdr_adjust_iovec(struct kvec *iov, __be32 *p)
  */
 extern void xdr_shift_buf(struct xdr_buf *, size_t);
 extern void xdr_buf_from_iov(struct kvec *, struct xdr_buf *);
-extern int xdr_buf_subsegment(struct xdr_buf *, struct xdr_buf *, unsigned int, unsigned int);
+extern int xdr_buf_subsegment(const struct xdr_buf *buf, struct xdr_buf *subbuf,
+			      unsigned int base, unsigned int len);
 extern void xdr_buf_trim(struct xdr_buf *, unsigned int);
 extern int read_bytes_from_xdr_buf(struct xdr_buf *, unsigned int, void *, unsigned int);
 extern int write_bytes_to_xdr_buf(struct xdr_buf *, unsigned int, void *, unsigned int);

net/sunrpc/xdr.c

@@ -1379,9 +1379,8 @@ EXPORT_SYMBOL_GPL(xdr_buf_from_iov);
  *
  * Returns -1 if base of length are out of bounds.
  */
-int
-xdr_buf_subsegment(struct xdr_buf *buf, struct xdr_buf *subbuf,
-			unsigned int base, unsigned int len)
+int xdr_buf_subsegment(const struct xdr_buf *buf, struct xdr_buf *subbuf,
+		       unsigned int base, unsigned int len)
 {
 	subbuf->buflen = subbuf->len = len;
 	if (base < buf->head[0].iov_len) {