[JFFS2] Introduce jffs2_link_node_ref() function to reduce code duplication

The same sequence of code was repeated in many places, to add a new
struct jffs2_raw_node_ref to an eraseblock and adjust the space accounting
accordingly. Move it out-of-line.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

diff --cc fs/jffs2/erase.c
Simple merge

diff --cc fs/jffs2/nodelist.c
Simple merge

diff --cc fs/jffs2/nodelist.h
Simple merge

diff --cc fs/jffs2/nodemgmt.c
Simple merge

diff --cc fs/jffs2/scan.c
Simple merge

diff --cc fs/jffs2/summary.c
Simple merge