net: socket: add __compat_sys_...msg() helpers; remove in-kernel calls to compat syscalls

Using the net-internal helpers __compat_sys_...msg() allows us to avoid
the internal calls to the compat_sys_...msg() syscalls.
compat_sys_recvmmsg() is handled in a different patch.

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

@@ -747,25 +747,48 @@ static unsigned char nas[21] = {
 };
 #undef AL
 
-COMPAT_SYSCALL_DEFINE3(sendmsg, int, fd, struct compat_msghdr __user *, msg, unsigned int, flags)
+static inline long __compat_sys_sendmsg(int fd,
+					struct compat_msghdr __user *msg,
+					unsigned int flags)
 {
 	return __sys_sendmsg(fd, (struct user_msghdr __user *)msg,
 			     flags | MSG_CMSG_COMPAT, false);
 }
 
+COMPAT_SYSCALL_DEFINE3(sendmsg, int, fd, struct compat_msghdr __user *, msg,
+		       unsigned int, flags)
+{
+	return __compat_sys_sendmsg(fd, msg, flags);
+}
+
+static inline long __compat_sys_sendmmsg(int fd,
+					 struct compat_mmsghdr __user *mmsg,
+					 unsigned int vlen, unsigned int flags)
+{
+	return __sys_sendmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
+			      flags | MSG_CMSG_COMPAT, false);
+}
+
 COMPAT_SYSCALL_DEFINE4(sendmmsg, int, fd, struct compat_mmsghdr __user *, mmsg,
 		       unsigned int, vlen, unsigned int, flags)
 {
-	return __sys_sendmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
-			      flags | MSG_CMSG_COMPAT, false);
+	return __compat_sys_sendmmsg(fd, mmsg, vlen, flags);
 }
 
-COMPAT_SYSCALL_DEFINE3(recvmsg, int, fd, struct compat_msghdr __user *, msg, unsigned int, flags)
+static inline long __compat_sys_recvmsg(int fd,
+					struct compat_msghdr __user *msg,
+					unsigned int flags)
 {
 	return __sys_recvmsg(fd, (struct user_msghdr __user *)msg,
 			     flags | MSG_CMSG_COMPAT, false);
 }
 
+COMPAT_SYSCALL_DEFINE3(recvmsg, int, fd, struct compat_msghdr __user *, msg,
+		       unsigned int, flags)
+{
+	return __compat_sys_recvmsg(fd, msg, flags);
+}
+
 static inline long __compat_sys_recvfrom(int fd, void __user *buf,
 					 compat_size_t len, unsigned int flags,
 					 struct sockaddr __user *addr,
@@ -893,13 +916,13 @@ COMPAT_SYSCALL_DEFINE2(socketcall, int, call, u32 __user *, args)
 					      compat_ptr(a[4]));
 		break;
 	case SYS_SENDMSG:
-		ret = compat_sys_sendmsg(a0, compat_ptr(a1), a[2]);
+		ret = __compat_sys_sendmsg(a0, compat_ptr(a1), a[2]);
 		break;
 	case SYS_SENDMMSG:
-		ret = compat_sys_sendmmsg(a0, compat_ptr(a1), a[2], a[3]);
+		ret = __compat_sys_sendmmsg(a0, compat_ptr(a1), a[2], a[3]);
 		break;
 	case SYS_RECVMSG:
-		ret = compat_sys_recvmsg(a0, compat_ptr(a1), a[2]);
+		ret = __compat_sys_recvmsg(a0, compat_ptr(a1), a[2]);
 		break;
 	case SYS_RECVMMSG:
 		ret = __compat_sys_recvmmsg(a0, compat_ptr(a1), a[2], a[3],