Move io/tst-fcntl temporary file creation to do_prepare.

This patch makes the io/tst-fcntl test create its temporary file in
do_prepare not do_test, so that add_temp_name is called in the parent
on the correct file name, as requested in the thread starting at
<https://sourceware.org/ml/libc-alpha/2015-10/msg00552.html>.  I don't
see any reason in this test that opening the file in the parent should
be problematic.

This patch is explicitly not exhaustive for such issues; other tests
still call add_temp_file in the child.  The general issue is noted at
<https://sourceware.org/glibc/wiki/Development_Todo/Master#Use_test-skeleton.c>.

Tested for x86_64 (that this test still passes and no longer leaves a
file behind in /tmp).

	* io/tst-fcntl.c (fd): New static variable.
	(do_prepare): Open temporary file here....
	(do_test): ...not here.