[JFFS2] Use rb_first() and rb_last() cleanup

Use rb_first() and rb_last() to implement frag_first() and frag_last().

Signed-off-by: Akinbou Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>