panic: add option to dump blocked tasks in panic_print

For debugging kernel panics and other bugs, there is already an option of
panic_print to dump all tasks' call stacks.  On today's large servers
running many containers, there could be thousands of tasks or more, and
this will print out huge amount of call stacks, taking a lot of time (for
serial console which is main target user case of panic_print).

And in many cases, only those several tasks being blocked are key for the
panic, so add an option to only dump blocked tasks' call stacks.

[akpm@linux-foundation.org: clarify documentation a little]
Link: https://lkml.kernel.org/r/20240202132042.3609657-1-feng.tang@intel.com
Signed-off-by: Feng Tang <feng.tang@intel.com>
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/sysctl/kernel.rst

@@ -853,6 +853,7 @@ bit 3  print locks info if ``CONFIG_LOCKDEP`` is on
 bit 4  print ftrace buffer
 bit 5  print all printk messages in buffer
 bit 6  print all CPUs backtrace (if available in the arch)
+bit 7  print only tasks in uninterruptible (blocked) state
 =====  ============================================
 
 So for example to print tasks and memory info on panic, user can::