submodule--helper: fix "sm_path" and other "module_cb_list" leaks

submodule--helper: fix "sm_path" and other "module_cb_list" leaks

Fix leaks in "struct module_cb_list" and the "struct module_cb" which
it contains, these fix leaks in e83e3333b57 (submodule: port submodule
subcommand 'summary' from shell to C, 2020-08-13).

The "sm_path" should always have been a "char *", not a "const
char *", we always create it with xstrdup().

We can't mark any tests passing passing with SANITIZE=leak using
"TEST_PASSES_SANITIZE_LEAK=true" as a result of this change, but
"t7401-submodule-summary.sh" gets closer to passing as a result of
this change.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>