fanotify: prepare for implicit event flags in mark mask

fanotify: prepare for implicit event flags in mark mask

So far, all flags that can be set in an fanotify mark mask can be set
explicitly by a call to fanotify_mark(2).

Prepare for defining implicit event flags that cannot be set by user with
fanotify_mark(2), similar to how inotify/dnotify implicitly set the
FS_EVENT_ON_CHILD flag.

Implicit event flags cannot be removed by user and mark gets destroyed
when only implicit event flags remain in the mask.

Link: https://lore.kernel.org/r/20200716084230.30611-7-amir73il@gmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>