ext3/jbd: Avoid WARN() messages when failing to write the superblock

This fixes a WARN backtrace in mark_buffer_dirty() that occurs during unmount
when the underlying block device is removed.  This bug has been seen on System
Z when removing all paths from a multipath-backed ext3 mount; on System P when
injecting enough PCI EEH errors to make the SCSI controller go offline; and
similar warnings have been seen (and patched) with ext2/ext4.

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 changes
have been made to ext4 by commit 914258bf

.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Jan Kara <jack@suse.cz>