net: socket: add __sys_setsockopt() helper; remove in-kernel call to syscall

Using the net-internal helper __sys_setsockopt() allows us to avoid the
internal calls to the sys_setsockopt() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

include/linux/socket.h

@@ -376,4 +376,5 @@ extern int __sys_socketpair(int family, int type, int protocol,
 			    int __user *usockvec);
 extern int __sys_shutdown(int fd, int how);
 
+
 #endif /* _LINUX_SOCKET_H */