mm/damon/dbgfs: check if rm_contexts input is for a real context

A user could write a name of a file under 'damon/' debugfs directory,
which is not a user-created context, to 'rm_contexts' file.  In the case,
'dbgfs_rm_context()' just assumes it's the valid DAMON context directory
only if a file of the name exist.  As a result, invalid memory access
could happen as below.  Fix the bug by checking if the given input is for
a directory.  This check can filter out non-context inputs because
directories under 'damon/' debugfs directory can be created via only
'mk_contexts' file.

This bug has found by syzbot[1].

[1] https://lore.kernel.org/damon/000000000000ede3ac05ec4abf8e@google.com/

Link: https://lkml.kernel.org/r/20221107165001.5717-2-sj@kernel.org
Fixes: 75c1c2b5

 ("mm/damon/dbgfs: support multiple contexts")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by:  <syzbot+6087eafb76a94c4ac9eb@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>	[5.15.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>