mm/damon/dbgfs: fix memory leak when using debugfs_lookup()

mm/damon/dbgfs: fix memory leak when using debugfs_lookup()

When calling debugfs_lookup() the result must have dput() called on it,
otherwise the memory will leak over time.  Fix this up by properly calling
dput().

Link: https://lkml.kernel.org/r/20220902191149.112434-1-sj@kernel.org
Fixes: 75c1c2b53c78b ("mm/damon/dbgfs: support multiple contexts")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>