jffs2: refactor csize usage in jffs2_do_read_inode_internal()

Replace the verbose `je32_to_cpu(latest_node->csize)' with a shorter
`csize'.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>