mptcp: fix conflict with <netinet/in.h>

Including <linux/mptcp.h> before the C library <netinet/in.h> header
causes symbol redefinition errors at compile-time due to duplicate
declarations and definitions in the <linux/in.h> header included by
<linux/mptcp.h>.

Explicitly include <netinet/in.h> before <linux/in.h> in
<linux/mptcp.h> when __KERNEL__ is not defined so that the C library
compatibility logic in <linux/libc-compat.h> is enabled when including
<linux/mptcp.h> in user space code.

Fixes: c11c5906 ("mptcp: add MPTCP_SUBFLOW_ADDRS getsockopt support")
Signed-off-by: Ossama Othman <ossama.othman@intel.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/mptcp.h

@@ -2,16 +2,17 @@
 #ifndef _UAPI_MPTCP_H
 #define _UAPI_MPTCP_H
 
+#ifndef __KERNEL__
+#include <netinet/in.h>		/* for sockaddr_in and sockaddr_in6	*/
+#include <sys/socket.h>		/* for struct sockaddr			*/
+#endif
+
 #include <linux/const.h>
 #include <linux/types.h>
 #include <linux/in.h>		/* for sockaddr_in			*/
 #include <linux/in6.h>		/* for sockaddr_in6			*/
 #include <linux/socket.h>	/* for sockaddr_storage and sa_family	*/
 
-#ifndef __KERNEL__
-#include <sys/socket.h>		/* for struct sockaddr			*/
-#endif
-
 #define MPTCP_SUBFLOW_FLAG_MCAP_REM		_BITUL(0)
 #define MPTCP_SUBFLOW_FLAG_MCAP_LOC		_BITUL(1)
 #define MPTCP_SUBFLOW_FLAG_JOIN_REM		_BITUL(2)