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