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

Using the net-internal helper __sys_sendto() allows us to avoid the
internal calls to the sys_sendto() 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

@@ -358,5 +358,8 @@ extern int __sys_sendmmsg(int fd, struct mmsghdr __user *mmsg,
 extern int __sys_recvfrom(int fd, void __user *ubuf, size_t size,
 			  unsigned int flags, struct sockaddr __user *addr,
 			  int __user *addr_len);
+extern int __sys_sendto(int fd, void __user *buff, size_t len,
+			unsigned int flags, struct sockaddr __user *addr,
+			int addr_len);
 
 #endif /* _LINUX_SOCKET_H */