syscall: avoid extra syscall on send/recvmsg on Linux

By simply rearranging the logic, we avoid the overhead of a superfluous
call to getsockopt. For, if p is already non empty, there's no point
in having to check if we need to attach dummy payload. This has
performance benefits when using send/recvmsg for high speed
communications.

Change-Id: Id85cff17328ecbf6d09dd52fbeeaa691dbe69b75
Reviewed-on: https://go-review.googlesource.com/108338
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed
tree: db549f58c987a30f011c1f7247eee934b85bf298
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  2. .github/
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  4. AUTHORS
  5. CONTRIBUTING.md
  6. CONTRIBUTORS
  7. LICENSE
  8. PATENTS
  9. README.md
  10. api/
  11. doc/
  12. favicon.ico
  13. lib/
  14. misc/
  15. robots.txt
  16. src/
  17. test/
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