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fsnotify_add_mark is supposed to add a mark to the g_list and i_list and to
set the group and inode for the mark.  fsnotify_destroy_mark_by_entry uses
the fact that ->group != NULL to know if this group should be destroyed or
if it's already been done.

But fsnotify_add_mark sets the group and inode before it actually adds the
mark to the i_list and g_list.  This can result in a race in inotify, it
requires 3 threads.

sys_inotify_add_watch("file")	sys_inotify_add_watch("file")	sys_inotify_rm_watch([a])
inotify_update_watch()
inotify_new_watch()
inotify_add_to_idr()
   ^--- returns wd = [a]
				inotfiy_update_watch()
				inotify_new_watch()
				inotify_add_to_idr()
				fsnotify_add_mark()
				   ^--- returns wd = [b]
				returns to userspace;
								inotify_idr_find([a])
								   ^--- gives us the pointer from task 1
fsnotify_add_mark()
   ^--- this is going to set the mark->group and mark->inode fields, but will
return -EEXIST because of...