cleanup: Adjust scoped_guard() macros to avoid potential warning

Change scoped_guard() and scoped_cond_guard() macros to make reasoning
about them easier for static analysis tools (smatch, compiler
diagnostics), especially to enable them to tell if the given usage of
scoped_guard() is with a conditional lock class (interruptible-locks,
try-locks) or not (like simple mutex_lock()).

Add compile-time error if scoped_cond_guard() is used for non-conditional
lock class.

Beyond easier tooling and a little shrink reported by bloat-o-meter
this patch enables developer to write code like:

int foo(struct my_drv *adapter)
{
	scoped_guard(spinlock, &adapter->some_spinlock)
		return adapter->spinlock_protected_var;
}

Current scoped_guard() implementation does not support that,
due to compiler complaining:
error: control reaches end of non-void function [-Werror=return-type]

Technical stuff about the change:
scoped_guard() macro uses common idiom of using "for" statement to declare
a scoped variable. Unfortunately, current logic is too hard for compiler
diagnostics to be sure that there is exactly one loop step; fix that.

To make any loop so trivial that there is no above warning, it must not
depend on any non-const variable to tell if there are more steps. There is
no obvious solution for that in C, but one could use the compound
statement expression with "goto" jumping past the "loop", effectively
leaving only the subscope part of the loop semantics.

More impl details:
one more level of macro indirection is now needed to avoid duplicating
label names;
I didn't spot any other place that is using the
"for (...; goto label) if (0) label: break;" idiom, so it's not packed for
reuse beyond scoped_guard() family, what makes actual macros code cleaner.

There was also a need to introduce const true/false variable per lock
class, it is used to aid compiler diagnostics reasoning about "exactly
1 step" loops (note that converting that to function would undo the whole
benefit).

Big thanks to Andy Shevchenko for help on this patch, both internal and
public, ranging from whitespace/formatting, through commit message
clarifications, general improvements, ending with presenting alternative
approaches - all despite not even liking the idea.

Big thanks to Dmitry Torokhov for the idea of compile-time check for
scoped_cond_guard() (to use it only with conditional locsk), and general
improvements for the patch.

Big thanks to David Lechner for idea to cover also scoped_cond_guard().

Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lkml.kernel.org/r/20241018113823.171256-1-przemyslaw.kitszel@intel.com

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