selftests: forwarding: mirror_gre_flower: Fix test result handling

The global variable RET needs to be initialized before each call to
log_test. This test case sets it once before running the tests, but then
calls log_tests for every individual test. Thus a failure in one of the
tests causes spurious failures in follow-up tests as well.

Fix by moving the initialization of RET from test_all() to
full_test_span_gre_dir_acl(), a function that implements the test.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>