cleanup: Basic compatibility with context analysis

Introduce basic compatibility with cleanup.h infrastructure.

We need to allow the compiler to see the acquisition and release of the
context lock at the start and end of a scope. However, the current
"cleanup" helpers wrap the lock in a struct passed through separate
helper functions, which hides the lock alias from the compiler (no
inter-procedural analysis).

While Clang supports scoped guards in C++, it's not possible to apply in
C code: https://clang.llvm.org/docs/ThreadSafetyAnalysis.html#scoped-context

However, together with recent improvements to Clang's alias analysis
abilities, idioms such as this work correctly now:

void spin_unlock_cleanup(spinlock_t **l) __releases(*l) { .. }
...
{
spinlock_t *lock_scope __cleanup(spin_unlock_cleanup) = &lock;
spin_lock(&lock); // lock through &lock
... critical section ...
} // unlock through lock_scope -[alias]-> &lock (no warnings)

To generalize this pattern and make it work with existing lock guards,
introduce DECLARE_LOCK_GUARD_1_ATTRS() and WITH_LOCK_GUARD_1_ATTRS().

These allow creating an explicit alias to the context lock instance that
is "cleaned" up with a separate cleanup helper. This helper is a dummy
function that does nothing at runtime, but has the release attributes to
tell the compiler what happens at the end of the scope.

Example usage:

DECLARE_LOCK_GUARD_1_ATTRS(mutex, __acquires(_T), __releases(*(struct mutex **)_T))
#define class_mutex_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(mutex, _T)

Note: To support the for-loop based scoped helpers, the auxiliary
variable must be a pointer to the "class" type because it is defined in
the same statement as the guard variable. However, we initialize it with
the lock pointer (despite the type mismatch, the compiler's alias
analysis still works as expected). The "_unlock" attribute receives a
pointer to the auxiliary variable (a double pointer to the class type),
and must be cast and dereferenced appropriately.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-7-elver@google.com