ext4: flush journal when switching from data=journal mode

It's necessary to flush the journal when switching away from
data=journal mode.  This is because there are no revoke records when
data blocks are journalled, but revoke records are required in the
other journal modes.

However, it is not necessary to flush the journal when switching into
data=journal mode, and flushing the journal is expensive.  So let's
avoid it in that case.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>