include/linux: fix excess fence.h kernel-doc notation

Fix excess fields in kernel-doc notation in <linux/fence.h>
after some struct fields were removed.

Fixes these kernel-doc warnings:
..//include/linux/fence.h:85: warning: Excess struct/union/enum/typedef member 'child_list' description in 'fence'
..//include/linux/fence.h:85: warning: Excess struct/union/enum/typedef member 'active_list' description in 'fence'

Fixes: 0431b906 ("staging/android: bring struct sync_pt back")

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>