mm/damon/dbgfs: print DAMON debugfs interface deprecation message

DAMON debugfs interface has announced to be deprecated after >v5.15 LTS
kernel is released.  And, v6.1.y has announced to be an LTS[1].

Though the announcement was there for a while, some people might not
noticed that so far.  Also, some users could depend on it and have
problems at  movng to the alternative (DAMON sysfs interface).

For such cases, warn DAMON debugfs interface deprecation with contacts
to ask helps when any DAMON debugfs interface file is opened.

[1] https://git.kernel.org/pub/scm/docs/kernel/website.git/commit/?id=332e9121320bc7461b2d3a79665caf153e51732c

[sj@kernel.org: split DAMON debugfs file open warning message, per Randy]
  Link: https://lkml.kernel.org/r/20230209192009.7885-4-sj@kernel.org
  Link: https://lkml.kernel.org/r/20230210044838.63723-4-sj@kernel.org
Link: https://lkml.kernel.org/r/20230209192009.7885-4-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>