Changes to make openzfs build within FreeBSD buildworld

A collection of header changes to enable FreeBSD to build
with vendored OpenZFS.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10635

lib/libspl/include/rpc/xdr.h

@@ -40,10 +40,13 @@
 
 #define	XDR_GET_BYTES_AVAIL 1
 
-typedef struct xdr_bytesrec {
+#ifndef HAVE_XDR_BYTESREC
+struct xdr_bytesrec {
 	bool_t xc_is_last_record;
 	size_t xc_num_avail;
-} xdr_bytesrec_t;
+};
+#endif
+typedef struct xdr_bytesrec  xdr_bytesrec_t;
 
 /*
  * This functionality is not required and is disabled in user space.