ip_tests.sh: fix errors

1) Eliminate '|| RC=$?' because because it may not be excecuted and the
   original value in RC can mess things up.  Also remove the RC=0
   initializations that are now unnecessary.
2) Exit test immediately with non-zero return code when a test fails.
3) Add missing parameters in calling tst_brk and tst_brkm

Signed-off-by: Simon Xu <xu.simon@oracle.com>
Acked-by: Jan Stancek <jstancek@redhat.com>