fanotify: fix flush of mntns marks

fanotify_mark(fd, FAN_MARK_FLUSH | FAN_MARK_MNTNS, ...) incorrectly
ends up causing removal inode marks.

Fixes: 0f46d81f ("fanotify: notify on mount attach and detach")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20250418193903.2607617-2-amir73il@gmail.com