ldp: return byte count from a successful recvfrom

Unless there's an error, recvfrom should return the number of bytes
received. The current code returns zero on success due to
ldp_copy_ep_to_sockaddr returning zero.

Change-Id: Ib843b99a08c44b5ee2d16efe9edab0cf5df65170
Signed-off-by: Haggai Eran <haggai.eran@gmail.com>