libceph: make ceph_tcp_connect() return int

There is no real need for ceph_tcp_connect() to return the socket
pointer it creates, since it already assigns it to con->sock, which
is visible to the caller.  Instead, have it return an error code,
which tidies things up a bit.

Signed-off-by: Alex Elder <elder@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>