return_to_param: don't modify memory on the stack

return_to_param: don't modify memory on the stack

The param_set/param_add code never tries to modify stack memory but when
we use the param_long_to_short mapping then it can happen.  What that looks
like is this:

	ptr = foo->bar;
	frob(foo);

The frob() function sets "foo->bar" and in the original code that would
change "ptr" as well, but "ptr" is stored on the stack and should stay
the same.

For PARAM_LIMIT and PARAM_FILTER then we do want to update "ptr".  That
looks like this:

	ptr = foo->bar;
	if (frob(foo)) { ...

Imagine that frob() only returns true if foo->bar is non-NULL then we know
that ptr is non-NULL also.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>