ext4: avoid issuing unnecessary barriers

(cherry picked from commit 6b17d902

)

We don't to issue an I/O barrier on an error or if we force commit
because we are doing data journaling.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andy Whitcroft <apw@canonical.com>