mm/damon/dbgfs: implement deprecation notice file

Implement a read-only file for DAMON debugfs interface deprecation notice,
to let users who manually read/write the DAMON debugfs files from their
shell command line easily notice the fact.

[arnd@arndb.de: fix bogus string length]
  Link: https://lkml.kernel.org/r/20240202124339.892862-1-arnd@kernel.org
Link: https://lkml.kernel.org/r/20240130013549.89538-4-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alex Shi <alexs@kernel.org>
Cc: Hu Haowen <2023002089@link.tyut.edu.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>