init.h: Disable sanitizer coverage for __init and __head

While __noinstr already contained __no_sanitize_coverage, it needs to
be added to __init and __head section markings to support the Clang
implementation of CONFIG_KSTACK_ERASE. This is to make sure the stack
depth tracking callback is not executed in unsupported contexts.

The other sanitizer coverage options (trace-pc and trace-cmp) aren't
needed in __head nor __init either ("We are interested in code coverage
as a function of a syscall inputs"[1]), so this is fine to disable for
them as well.

Link: https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/kcov.c?h=v6.14#n179

 [1]
Acked-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/r/20250724055029.3623499-3-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>