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

Using the net-internal helper __sys_shutdown() allows us to avoid the
internal calls to the sys_shutdown() 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

@@ -374,5 +374,6 @@ extern int __sys_getpeername(int fd, struct sockaddr __user *usockaddr,
 			     int __user *usockaddr_len);
 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/compat.c

@@ -849,7 +849,7 @@ COMPAT_SYSCALL_DEFINE2(socketcall, int, call, u32 __user *, args)
 					  compat_ptr(a[4]), compat_ptr(a[5]));
 		break;
 	case SYS_SHUTDOWN:
-		ret = sys_shutdown(a0, a1);
+		ret = __sys_shutdown(a0, a1);
 		break;
 	case SYS_SETSOCKOPT:
 		ret = compat_sys_setsockopt(a0, a1, a[2],

net/socket.c

@@ -1942,7 +1942,7 @@ SYSCALL_DEFINE5(getsockopt, int, fd, int, level, int, optname,
  *	Shutdown a socket.
  */
 
-SYSCALL_DEFINE2(shutdown, int, fd, int, how)
+int __sys_shutdown(int fd, int how)
 {
 	int err, fput_needed;
 	struct socket *sock;
@@ -1957,6 +1957,11 @@ SYSCALL_DEFINE2(shutdown, int, fd, int, how)
 	return err;
 }
 
+SYSCALL_DEFINE2(shutdown, int, fd, int, how)
+{
+	return __sys_shutdown(fd, how);
+}
+
 /* A couple of helpful macros for getting the address of the 32/64 bit
  * fields which are the same type (int / unsigned) on our platforms.
  */
@@ -2544,7 +2549,7 @@ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
 				     (int __user *)a[5]);
 		break;
 	case SYS_SHUTDOWN:
-		err = sys_shutdown(a0, a1);
+		err = __sys_shutdown(a0, a1);
 		break;
 	case SYS_SETSOCKOPT:
 		err = sys_setsockopt(a0, a1, a[2], (char __user *)a[3], a[4]);