tracing/histograms: Allow variables to have some modifiers

commit e30fbc61 upstream.

Modifiers are used to change the behavior of keys. For instance, they
can grouped into buckets, converted to syscall names (from the syscall
identifier), show task->comm of the current pid, be an array of longs
that represent a stacktrace, and more.

It was found that nothing stopped a value from taking a modifier. As
values are simple counters. If this happened, it would call code that
was not expecting a modifier and crash the kernel. This was fixed by
having the ___create_val_field() function test if a modifier was present
and fail if one was. This fixed the crash.

Now there's a problem with variables. Variables are used to pass fields
from one event to another. Variables are allowed to have some modifiers,
as the processing may need to happen at the time of the event (like
stacktraces and comm names of the current pid). The issue is that it too
uses __create_val_field(). Now that fails on modifiers, variables can no
longer use them (this is a regression).

As not all modifiers are for variables, have them use a separate check.

Link: https://lore.kernel.org/linux-trace-kernel/20230523221108.064a5d82@rorschach.local.home

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Fixes: e0213434

 ("tracing: Do not let histogram values have some modifiers")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>