xdr: header cleanup

#16047 notes that include/os/freebsd/spl/rpc/xdr.h carried an
(apparently) incompatible license. While looking into it, it seems that
this file is actually unnecessary these days - FreeBSD's kernel XDR has
XDR_CONTROL, xdrmem_control and XDR_GET_BYTES_AVAIL, while userspace has
XDR_CONTROL and xdrmem_control, and our implementation of
XDR_GET_BYTES_AVAIL for libspl works nicely with it. So this removes
that file outright.

To keep the includes in nvpair.c tidy, I've made a few small adjustments
to the Linux headers. By definition, rpc/types.h provides bool_t and is
included before rpc/xdr.h, so I've created rpc/types.h for Linux. This
isn't necessary for userspace; both FreeBSD native and tirpc on Linux
already have these headers set up correctly.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16047 
Closes #16051

include/os/linux/spl/rpc/types.h

@@ -0,0 +1,30 @@
+/*
+ *  Copyright (c) 2008 Sun Microsystems, Inc.
+ *  Written by Ricardo Correia <Ricardo.M.Correia@Sun.COM>
+ *
+ *  This file is part of the SPL, Solaris Porting Layer.
+ *
+ *  The SPL is free software; you can redistribute it and/or modify it
+ *  under the terms of the GNU General Public License as published by the
+ *  Free Software Foundation; either version 2 of the License, or (at your
+ *  option) any later version.
+ *
+ *  The SPL is distributed in the hope that it will be useful, but WITHOUT
+ *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ *  for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef _SPL_RPC_TYPES_H
+#define	_SPL_RPC_TYPES_H
+
+#include <sys/types.h>
+
+/* Just enough to support rpc/xdr.h */
+
+typedef int bool_t;
+
+#endif /* SPL_RPC_TYPES_H */

include/os/linux/spl/rpc/xdr.h

@@ -23,8 +23,6 @@
 
 #include <sys/types.h>
 
-typedef int bool_t;
-
 /*
  * XDR enums and types.
  */