net: add scheduling point in recvmmsg/sendmmsg

Applications often have to reduce number of datagrams
they receive or send per system call to avoid starvation problems.

Really the kernel should take care of this by using cond_resched(),
so that applications can experiment bigger batch sizes.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/socket.c

@@ -2041,6 +2041,7 @@ int __sys_sendmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 		if (err)
 			break;
 		++datagrams;
+		cond_resched();
 	}
 
 	fput_light(sock->file, fput_needed);
@@ -2236,6 +2237,7 @@ int __sys_recvmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 		/* Out of band data, return right away */
 		if (msg_sys.msg_flags & MSG_OOB)
 			break;
+		cond_resched();
 	}
 
 out_put: