dnotify: replace dnotify_mark_mutex with dnotify_group mutex

Since we introduced a new mutex for fsnotify groups we dont need an extra
dnotify_mark_mutex any more.
This patch replaces the dnotify_mark_mutex with the dnotify_group mutex.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Eric Paris <eparis@redhat.com>