ext4: write superblock only once on unmount

In some rather rare cases it is possible that ext4 may the superblock
to the media twice. This patch makes sure this does not happen. This
should speed up unmounting in those cases.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>