samples: add hung_task detector mutex blocking sample

Add a hung_task detector mutex blocking test sample code.

This module will create a dummy file on the debugfs.  That file will cause
the read process to sleep for enough long time (256 seconds) while holding
a mutex.  As a result, the second process will wait on the mutex for a
prolonged duration and be detected by the hung_task detector.

Usage is;

 > cd /sys/kernel/debug/hung_task
 > cat mutex & cat mutex

and wait for hung_task message.

[akpm@linux-foundation.org: make `hung_task_dir' static]
  Closes: https://lore.kernel.org/oe-kbuild-all/202503180827.4StpuFrD-lkp@intel.com/
Link: https://lkml.kernel.org/r/174046696281.2194069.4567490148001547311.stgit@mhiramat.tok.corp.google.com
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Anna Schumaker <anna.schumaker@oracle.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Granados <joel.granados@kernel.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yongliang Gao <leonylgao@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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