jffs2: remove unnecessary GC pass on umount

jffs2: remove unnecessary GC pass on umount

We do not need to call 'jffs2_write_super()' on unmount. 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 unmount 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 8c85e125124a473d6f3e9bb187b0b84207f81d91.

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