Use rb_first() and rb_last() to implement frag_first() and frag_last().
Signed-off-by: NAkinbou Mita <akinobu.mita@gmail.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>