mm/damon/core: nullify pointer ctx->kdamond with a NULL

mm/damon/core: nullify pointer ctx->kdamond with a NULL

Currently a plain integer is being used to nullify the pointer
ctx->kdamond.  Use NULL instead.  Cleans up sparse warning:

  mm/damon/core.c:317:40: warning: Using plain integer as NULL pointer

Link: https://lkml.kernel.org/r/20210925215908.181226-1-colin.king@canonical.com
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>