mount/conf: avoid creating temporary object

mount/conf: avoid creating temporary object

instead of creating an empty string and then mutating it, just
assign the return value of `eaddr.ip_n_port_to_str()` to it.
as C++17 enforces copy ellision in this case, we can avoid
two unnecessary calls here:

- creating a temporary object and destroying it.
- memcpy the temporary object.

also, after this change, the code is simpler.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>