jffs2: stack dump on sync

[Frank Mori Hess]

Hi, I'm using kernel 4.1.17 and I noticed jffs2 dumping stacktraces on
sync, as reported here:

https://patchwork.ozlabs.org/patch/298933/#689224

As far as I can tell, this has not been fixed in newer versions
although I haven't tested them.  It seems the problem occurs when
CONFIG_JFFS2_FS_WRITEBUFFER is enabled, which causes jffs2_sync_fs()
in jffs2/super.c to call

cancel_delayed_work_sync(&c->wbuf_dwork);

However, wbuf_dwork is generally not initialized, producing the stack dump.

-- 
Frank