net: stop multiple sends into single capacity channel in lookupIP

ch is of size 1, and has only one read. But current code can
write to ch more than once. This makes goroutines that do network
name lookups block forever. Only 500 goroutines are allowed, and
we eventually run out of goroutines.

Rewrite the code to only write into ch once.

Fixes #24178

Change-Id: Ifbd37db377c8b05e69eca24cc9147e7f86f899d8
Reviewed-on: https://go-review.googlesource.com/111718


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>