fsnotify: consistent behavior for parent not watching children

The logic for handling events on child in groups that have a mark on
the parent inode, but without FS_EVENT_ON_CHILD flag in the mask is
duplicated in several places and inconsistent.

Move the logic into the preparation of mark type iterator, so that the
parent mark type will be excluded from all mark type iterations in that
case.

This results in several subtle changes of behavior, hopefully all
desired changes of behavior, for example:

- Group A has a mount mark with FS_MODIFY in mask
- Group A has a mark with ignore mask that does not survive FS_MODIFY
  and does not watch children on directory D.
- Group B has a mark with FS_MODIFY in mask that does watch children
  on directory D.
- FS_MODIFY event on file D/foo should not clear the ignore mask of
  group A, but before this change it does

And if group A ignore mask was set to survive FS_MODIFY:
- FS_MODIFY event on file D/foo should be reported to group A on account
  of the mount mark...