Add userspace notification for MGLRU generation creation

Add sysfs_notify in inc_max_seq function so that every time a new MGLRU
generation is created so that user space programs can notice that by
polling /sys/kernel/mm/lru_gen/admin.

BUG=b:245255700
TEST=Follow these steps:
- Open /sys/kernel/mm/lru_gen/admin and read it once.
- poll the fd you opend and wait for the POLLPRI event.
- Put some memory pressure on the kernel and see if the poll ever
  returns.

Signed-off-by: Koki Ryu <kokiryu@google.com>

Change-Id: I11023f002ecf235a9933bb87fbd3183e9b08a9ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3936130


Commit-Queue: Kameron Lutes <kalutes@chromium.org>
Tested-by: Koki Ryu <kokiryu@chromium.org>
Reviewed-by: Charles William Dick <cwd@google.com>
Reviewed-by: Kameron Lutes <kalutes@chromium.org>
Signed-off-by: Richard Fung <richardfung@google.com>