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 <ldv@altlinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>