libio: Update internal fmemopen position after write (BZ #20005)

Current GLIBC fmemopen fails with a simple testcase:

  char buffer[500] = "x";
  FILE *stream;
  stream = fmemopen(buffer, 500, "r+");
  fwrite("fish",sizeof(char),5,stream);
  printf("pos-1:%ld\n",ftell(stream));
  fflush(stream);
  printf("pos-2:%ld\n",ftell(stream));

It returns:

  pos-1:5
  pos-2:0

Where it should return:

  pos-1:5
  pos-2:5

This is due the internal write function does not correctly update the internal
object position state and then the seek operation returns a wrong value.  This
patch fixes it.

It fixes both BZ #20005 and BZ #19230 (marked as duplicated). A new test is
added to check for such case.

Tested on x86_64 and i686.

This is a backport of f9123b50.

	* libio/fmemopen.c (fmemopen_write): Update internal position after
	write.
	* stdio-common/Makefile (tests): Add tst-fmemopen4.c.
	* stdio-common/tst-fmemopen4.c: New file..