diag: diagfwd_socket: Increase socket receive buf

Diag expects heavy traffic when maximum logging is enabled. The maximum
amount of packets QRTR will deliver before flow control kicks in is
16k * 10. Use setsockopt to increase the rcvbuf for the diag sockets
to avoid dropping any packets from the remote.

Change-Id: I782997c00e1f4ccefb8cee635b65ae2edb733396
Signed-off-by: Chris Lew <clew@codeaurora.org>