uapi: fix linux/if.h userspace compilation errors

Include <sys/socket.h> (guarded by ifndef __KERNEL__) to fix
the following linux/if.h userspace compilation errors:

/usr/include/linux/if.h:234:19: error: field 'ifru_addr' has incomplete type
struct sockaddr ifru_addr;
/usr/include/linux/if.h:235:19: error: field 'ifru_dstaddr' has incomplete type
struct sockaddr ifru_dstaddr;
/usr/include/linux/if.h:236:19: error: field 'ifru_broadaddr' has incomplete type
struct sockaddr ifru_broadaddr;
/usr/include/linux/if.h:237:19: error: field 'ifru_netmask' has incomplete type
struct sockaddr ifru_netmask;
/usr/include/linux/if.h:238:20: error: field 'ifru_hwaddr' has incomplete type
struct sockaddr ifru_hwaddr;

This also fixes userspace compilation of the following uapi headers:
linux/atmbr2684.h
linux/gsmmux.h
linux/if_arp.h
linux/if_bonding.h
linux/if_frad.h
linux/if_pppox.h
linux/if_tunnel.h
linux/netdevice.h
linux/route.h
linux/wireless.h

As no uapi header provides a definition of struct sockaddr, inclusion
of <sys/socket.h> seems to be the most conservative and the only safe
fix available.

All current users of <linux/if.h> are very likely to be including
<sys/socket.h> already because the latter is the sole provider
of struct sockaddr definition in libc, so adding a uapi header
with a definition of struct sockaddr would create a potential
conflict with <sys/socket.h>.

Replacing struct sockaddr in the definition of struct ifreq with
a different type would create a potential incompatibility with current
users of struct ifreq who might rely on ifru_addr et al members being
of type struct sockaddr.

Signed-off-by: Dmitry V. Levin <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/if.h

···
 #include <linux/socket.h>		/* for "struct sockaddr" et al	*/
 #include <linux/compiler.h>		/* for "__user" et al           */
 
+#ifndef __KERNEL__
+#include <sys/socket.h>			/* for struct sockaddr.		*/
+#endif
+
 #if __UAPI_DEF_IF_IFNAMSIZ
 #define	IFNAMSIZ	16
 #endif /* __UAPI_DEF_IF_IFNAMSIZ */