More minor style fixes.

Author: aardvark179

src/main/ruby/truffleruby/core/io.rb

@@ -1660,7 +1660,7 @@ def nonblock=(value)
                 else
                   old_flags & ~NONBLOCK
                 end
-    fcntl(F_SETFL, new_flags) if old_flags != new_flags
+    fcntl(F_SETFL, new_flags) unless old_flags == new_flags
     self
   end
 
@@ -1841,7 +1841,7 @@ def read_nonblock(size, buffer = nil, exception: true)
       str
     when String
       buffer ? buffer.replace(str) : str
-    else
+    else # EOF
       if exception
         raise EOFError, 'end of file reached'
       else

src/main/ruby/truffleruby/core/posix.rb

@@ -373,7 +373,7 @@ def self.read_string_nonblock(io, count, exception)
       string
     elsif TRY_AGAIN_ERRNOS.include? errno
       raise IO::EAGAINWaitReadable if exception
-      return :wait_readable
+      :wait_readable
     else
       Errno.handle_errno(errno)
     end