fsnotify: allow addition of duplicate fsnotify marks

This patch allows a task to add a second fsnotify mark to an inode for the
same group.  This mark will be added to the end of the inode's list and
this will never be found by the stand fsnotify_find_mark() function.   This
is useful if a user wants to add a new mark before removing the old one.

Signed-off-by: Eric Paris <eparis@redhat.com>