staging: lustre: selftest: freeing an error pointer

We should just return directly if memdup_user() fails. The current code
tries to free "param" which is an error pointer so it will Oops.

Fixes: 2baddf26 ("staging: lustre: use memdup_user to allocate memory and copy from user")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>