ext2: clear uptodate flag on super block I/O error

This fixes a WARN backtrace in mark_buffer_dirty() that occurs during
unmount when a USB or floppy device is removed. I reported this a kernel
regression, but looks like it might have been there for longer
than that.

The super block update from a previous operation has marked the buffer
as in error, and the flag has to be cleared before doing the update.
(Similar code already exists in ext4).

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jan Kara <jack@suse.cz>