jffs2: remove unnecessary GC pass on sync

jffs2: remove unnecessary GC pass on sync

We do not need to call 'jffs2_write_super()' on sync. This function
causes a GC pass to make sure the current contents is pushed out with
the data which we already have on the media.

But this is not needed on unmount and only slows sync down unnecessarily.
It is enough to just sync the write-buffer.

This call was added by one of the generic VFS rework patch-sets,
see 4b2f9ee0c949e60f4cfcd2cbe13c0d41b91c68ca.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>