x86: add missing const qualifiers for log_lvl

x86: add missing const qualifiers for log_lvl

Currently, the log-level of show_stack() depends on a platform
realization.  It creates situations where the headers are printed with
lower log level or higher than the stacktrace (depending on a platform or
user).

Furthermore, it forces the logic decision from user to an architecture
side.  In result, some users as sysrq/kdb/etc are doing tricks with
temporary rising console_loglevel while printing their messages.  And in
result it not only may print unwanted messages from other CPUs, but also
omit printing at all in the unlucky case where the printk() was deferred.

Introducing log-level parameter and KERN_UNSUPPRESSED [1] seems an easier
approach than introducing more printk buffers.  Also, it will consolidate
printings with headers.

Keep log_lvl const show_trace_log_lvl() and printk_stack_address() as the
new generic show_stack_loglvl() wants to have a proper const qualifier.

And gcc rightfully produces warnings in case it's not keept:
arch/x86/kernel/dumpstack.c: In function `show_stack':
arch/x86/kernel/dumpstack.c:294:37: warning: passing argument 4 of `show_trace_log_lv ' discards `const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  294 |  show_trace_log_lvl(task, NULL, sp, loglvl);
      |                                     ^~~~~~
arch/x86/kernel/dumpstack.c:163:32: note: expected `char *' but argument is of type `const char *'
  163 |    unsigned long *stack, char *log_lvl)
      |                          ~~~~~~^~~~~~~

[1]: https://lore.kernel.org/lkml/20190528002412.1625-1-dima@arista.com/T/#u

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20200418201944.482088-41-dima@arista.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>