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

Using the net-internal helper __sys_socketpair() allows us to avoid the
internal calls to the sys_socketpair() 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/compat.c

@@ -832,7 +832,7 @@ COMPAT_SYSCALL_DEFINE2(socketcall, int, call, u32 __user *, args)
 		ret = __sys_getpeername(a0, compat_ptr(a1), compat_ptr(a[2]));
 		break;
 	case SYS_SOCKETPAIR:
-		ret = sys_socketpair(a0, a1, a[2], compat_ptr(a[3]));
+		ret = __sys_socketpair(a0, a1, a[2], compat_ptr(a[3]));
 		break;
 	case SYS_SEND:
 		ret = sys_send(a0, compat_ptr(a1), a[2], a[3]);