[JFFS2] Fix build failure with !CONFIG_JFFS2_FS_WRITEBUFFER

Build failure introduced by 5bf17237


[JFFS2] Write buffer offset adjustment for NOR-ECC (Sibley) flash

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>