fanotify: fix logic of reporting name info with watched parent

fanotify: fix logic of reporting name info with watched parent

The victim inode's parent and name info is required when an event
needs to be delivered to a group interested in filename info OR
when the inode's parent is interested in an event on its children.

Let us call the first condition 'parent_needed' and the second
condition 'parent_interested'.

In fsnotify_parent(), the condition where the inode's parent is
interested in some events on its children, but not necessarily
interested the specific event is called 'parent_watched'.

fsnotify_parent() tests the condition (!parent_watched && !parent_needed)
for sending the event without parent and name info, which is correct.

It then wrongly assumes that parent_watched implies !parent_needed
and tests the condition (parent_watched && !parent_interested)
for sending the event without parent and name info, which is wrong,
because parent may still be needed by some group.

For example, after initializing a group with FAN_REPORT_DFID_NAME and
adding a FAN_MARK_MOUNT with FAN_OPEN mask, open events on non-directory
children of "testdir" are delivered with file name info.

After adding another mark to the same group on the parent "testdir"
with FAN_CLOSE|FAN_EVENT_ON_CHILD mask, open events on non-directory
children of "testdir" are no longer delivered with file name info.

Fix the logic and use auxiliary variables to clarify the conditions.

Fixes: 9b93f33105f5 ("fsnotify: send event with parent/name info to sb/mount/non-dir marks")
Cc: stable@vger.kernel.org#v5.9
Link: https://lore.kernel.org/r/20201108105906.8493-1-amir73il@gmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>