mesa: Disallow relinking if a program is used by an active XFB object.

mesa: Disallow relinking if a program is used by an active XFB object.

Paused transform feedback objects may refer to a program other than the
current program.  If any active objects refer to a program, LinkProgram
must reject the request to relink.

The code to detect this is ugly since _mesa_HashWalk is awkward to use,
but unfortunately we can't use hash_table_foreach since there's no way
to get at the underlying struct hash_table (and even then, we'd need to
handle locking somehow).

Fixes the last subcase of Piglit's new ARB_transform_feedback2
api-errors test.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>