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 */

net/socket.c

@@ -1878,8 +1878,8 @@ SYSCALL_DEFINE4(recv, int, fd, void __user *, ubuf, size_t, size,
  *	to pass the user mode parameter for the protocols to sort out.
  */
 
-SYSCALL_DEFINE5(setsockopt, int, fd, int, level, int, optname,
-		char __user *, optval, int, optlen)
+static int __sys_setsockopt(int fd, int level, int optname,
+			    char __user *optval, int optlen)
 {
 	int err, fput_needed;
 	struct socket *sock;
@@ -1907,6 +1907,12 @@ SYSCALL_DEFINE5(setsockopt, int, fd, int, level, int, optname,
 	return err;
 }
 
+SYSCALL_DEFINE5(setsockopt, int, fd, int, level, int, optname,
+		char __user *, optval, int, optlen)
+{
+	return __sys_setsockopt(fd, level, optname, optval, optlen);
+}
+
 /*
  *	Get a socket option. Because we don't know the option lengths we have
  *	to pass a user mode parameter for the protocols to sort out.
@@ -2552,7 +2558,8 @@ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
 		err = __sys_shutdown(a0, a1);
 		break;
 	case SYS_SETSOCKOPT:
-		err = sys_setsockopt(a0, a1, a[2], (char __user *)a[3], a[4]);
+		err = __sys_setsockopt(a0, a1, a[2], (char __user *)a[3],
+				       a[4]);
 		break;
 	case SYS_GETSOCKOPT:
 		err =