test_io.rb: defer f_type

* test/ruby/test_io.rb (test_seek, test_seek_symwhence): defer
  File::Statfs#type call which may not be implemented, to mitigate
  errors on platforms where SEEK_DATA is available but f_type in
  struct statfs is not.  [ruby-dev:48154] [Bug #9789]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45783 b2dd03c8-39d4-4d8f-98ff-823fe69b080e