reiserfs: make reiserfs default to barrier=flush

Change the default reiserfs mount option to barrier=flush.  Based on a patch
from Jeff Mahoney in the SuSE tree.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>