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

Using the net-internal helper __sys_getsockopt() allows us to avoid the
internal calls to the sys_getsockopt() 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>

net/socket.c

@@ -1918,8 +1918,8 @@ SYSCALL_DEFINE5(setsockopt, int, fd, int, level, int, optname,
  *	to pass a user mode parameter for the protocols to sort out.
  */
 
-SYSCALL_DEFINE5(getsockopt, int, fd, int, level, int, optname,
-		char __user *, optval, int __user *, optlen)
+static int __sys_getsockopt(int fd, int level, int optname,
+			    char __user *optval, int __user *optlen)
 {
 	int err, fput_needed;
 	struct socket *sock;
@@ -1944,6 +1944,12 @@ SYSCALL_DEFINE5(getsockopt, int, fd, int, level, int, optname,
 	return err;
 }
 
+SYSCALL_DEFINE5(getsockopt, int, fd, int, level, int, optname,
+		char __user *, optval, int __user *, optlen)
+{
+	return __sys_getsockopt(fd, level, optname, optval, optlen);
+}
+
 /*
  *	Shutdown a socket.
  */
@@ -2563,7 +2569,7 @@ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
 		break;
 	case SYS_GETSOCKOPT:
 		err =
-		    sys_getsockopt(a0, a1, a[2], (char __user *)a[3],
+		    __sys_getsockopt(a0, a1, a[2], (char __user *)a[3],
 				     (int __user *)a[4]);
 		break;
 	case SYS_SENDMSG: