ifa_maintain_loopback_route: adjust debugging output

ifa_maintain_loopback_route: adjust debugging output

Correction after r333476:

- write this as LOG_DEBUG again instead of LOG_INFO;
- get back function name into the message;
- error may be ESRCH if an address is removed in process (by carp f.e.),
not only ENOENT;
- expression complexity grows, so try making it more readable.

MFC after:	1 week