arm: add show_stack_loglvl()

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.

Introduce show_stack_loglvl(), that eventually will substitute
show_stack().

[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: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will@kernel.org>
Link: http://lkml.kernel.org/r/20200418201944.482088-9-dima@arista.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm/kernel/traps.c

@@ -247,10 +247,16 @@ static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
 }
 #endif
 
+void show_stack_loglvl(struct task_struct *tsk, unsigned long *sp,
+		       const char *loglvl)
+{
+	dump_backtrace(NULL, tsk, loglvl);
+	barrier();
+}
+
 void show_stack(struct task_struct *tsk, unsigned long *sp)
 {
-	dump_backtrace(NULL, tsk, KERN_DEFAULT);
-	barrier();
+	show_stack_loglvl(tsk, sp, KERN_DEFAULT);
 }
 
 #ifdef CONFIG_PREEMPT