net: socket: add __sys_accept4() helper; remove in-kernel call to syscall

Using the net-internal helper __sys_accept4() allows us to avoid the
internal calls to the sys_accept4() 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>
diff --git a/net/compat.c b/net/compat.c
index fc82982..0ff9f74 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -823,7 +823,7 @@
 		ret = sys_listen(a0, a1);
 		break;
 	case SYS_ACCEPT:
-		ret = sys_accept4(a0, compat_ptr(a1), compat_ptr(a[2]), 0);
+		ret = __sys_accept4(a0, compat_ptr(a1), compat_ptr(a[2]), 0);
 		break;
 	case SYS_GETSOCKNAME:
 		ret = sys_getsockname(a0, compat_ptr(a1), compat_ptr(a[2]));
@@ -873,7 +873,7 @@
 					  compat_ptr(a[4]));
 		break;
 	case SYS_ACCEPT4:
-		ret = sys_accept4(a0, compat_ptr(a1), compat_ptr(a[2]), a[3]);
+		ret = __sys_accept4(a0, compat_ptr(a1), compat_ptr(a[2]), a[3]);
 		break;
 	default:
 		ret = -EINVAL;