io.c: check socket on other than linux

* io.c (nogvl_copy_stream_sendfile): check socket on other than
  linux, as sendfile(2) on non-socket fd works only on linux.
  [Feature #9427]

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44750 b2dd03c8-39d4-4d8f-98ff-823fe69b080e