After prior kernel change, mmap() on TCP socket only reserves VMA.

We have to use getsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...)
to perform the transfert of pages from skbs in TCP receive queue into such VMA.

struct tcp_zerocopy_receive {
	__u64 address;		/* in: address of mapping */
	__u32 length;		/* in/out: number of bytes to map/mapped */
	__u32 recv_skip_hint;	/* out: amount of bytes to skip */
};

After a successful getsockopt(...TCP_ZEROCOPY_RECEIVE...), @length contains
number of bytes that were mapped, and @recv_skip_hint contains number of bytes
that should be read using conventional read()/recv()/recvmsg() system calls,
to skip a sequence of bytes that can not be mapped, because not properly page
aligned.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>