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 8c85e125.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>