[JFFS2] Use f->target instead of f->dents for symlink target

JFFS2 uses f->dents to store the pointer to the symlink target string (in case
the inode is symlink). This is somewhat ugly to use the same field for
different reasons. Introduce distinct field f->target for this purpose.
Note, f->fragtree, f->dents, f->target may probably be put in a union.

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