[JFFS2] Move scattered function into related files

Move functions to read inodes into readinode.c
Move functions to handle fragtree and dentry lists into nodelist.[ch]

Signed-off-by: Artem B. Bityutskiy <dedekind@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>