Fix theoretical leak of netconfig(3) resources in svcunix_create(..)

In the event that the getconfig(3) call in svcunix_create is partly successful,
some of the netconfig(3) resources allocated might be leaked if the call returns
NULL as endnetconfig(3) wasn't called explicitly in that case. Ensure that the
resources are fully cleaned up by going to the done label, which will call
endnetconfig(3) for us.

MFC after: 1 week
Reported by: Coverity
Submitted by: Miles Ohlrich <miles.ohlrich@isilon.com>
Sponsored by: EMC / Isilon Storage Division