net: pass a function pointer to check if need retry

This is a prepare patch for 'retry write'.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: retry write for EAGAIN

Since we set timeout for write, we'll get EAGAIN even for blocking sockfd.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: pass epoch to sheep_exec_req()

Let all the proceduers to enjoy retry.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>