xattr: guard against simultaneous glibc header inclusion

If the glibc xattr.h header is included after the uapi header,
compilation fails due to an enum re-using a #define from the uapi
header.

Protect against this by guarding the define and enum inclusions against
each other.

(See https://lists.debian.org/debian-glibc/2014/03/msg00029.html
and https://sourceware.org/glibc/wiki/Synchronizing_Headers
for more information.)
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Allan McRae <allan@archlinux.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/uapi/linux/libc-compat.h

@@ -85,6 +85,12 @@
 
 #endif /* _NETINET_IN_H */
 
+/* Definitions for xattr.h */
+#if defined(_SYS_XATTR_H)
+#define __UAPI_DEF_XATTR		0
+#else
+#define __UAPI_DEF_XATTR		1
+#endif
 
 /* If we did not see any headers from any supported C libraries,
  * or we are being included in the kernel, then define everything
@@ -98,6 +104,9 @@
 #define __UAPI_DEF_IPV6_MREQ		1
 #define __UAPI_DEF_IPPROTO_V6		1
 
+/* Definitions for xattr.h */
+#define __UAPI_DEF_XATTR		1
+
 #endif /* __GLIBC__ */
 
 #endif /* _UAPI_LIBC_COMPAT_H */

include/uapi/linux/xattr.h

@@ -7,11 +7,18 @@
   Copyright (c) 2001-2002 Silicon Graphics, Inc.  All Rights Reserved.
   Copyright (c) 2004 Red Hat, Inc., James Morris <jmorris@redhat.com>
 */
+
+#include <linux/libc-compat.h>
+
 #ifndef _UAPI_LINUX_XATTR_H
 #define _UAPI_LINUX_XATTR_H
 
+#ifdef __UAPI_DEF_XATTR
+#define __USE_KERNEL_XATTR_DEFS
+
 #define XATTR_CREATE	0x1	/* set value, fail if attr already exists */
 #define XATTR_REPLACE	0x2	/* set value, fail if attr does not exist */
+#endif
 
 /* Namespaces */
 #define XATTR_OS2_PREFIX "os2."