[JFFS2] Switch to using an array of jffs2_raw_node_refs instead of a list.

This allows us to drop another pointer from the struct jffs2_raw_node_ref,
shrinking it to 8 bytes on 32-bit machines (if the TEST_TOTLEN) paranoia
check is turned off, which will be committed soon).

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