tcp-zerocopy: Return inq along with tcp receive zerocopy.

This patchset is intended to reduce the number of extra system calls
imposed by TCP receive zerocopy. For ping-pong RPC style workloads,
this patchset has demonstrated a system call reduction of about 30%
when coupled with userspace changes.

For applications using edge-triggered epoll, returning inq along with
the result of tcp receive zerocopy could remove the need to call
recvmsg()=-EAGAIN after a successful zerocopy. Generally speaking,
since normally we would need to perform a recvmsg() call for every
successful small RPC read via TCP receive zerocopy, returning inq can
reduce the number of system calls performed by approximately half.

Signed-off-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>