ext4: don't show mount options in /proc/mounts if there is no journal

After creating an ext4 file system without a journal:

  # mke2fs -t ext4 -O ^has_journal /dev/sda
  # mount -t ext4 /dev/sda /test

the /proc/mounts will show:
"/dev/sda /test ext4 rw,relatime,user_xattr,acl,barrier=1,data=writeback 0 0"
which can fool users into thinking that the fs is using writeback mode.

So don't set the writeback option when the journal has not been
enabled; we don't depend on the writeback option being set, since
ext4_should_writeback_data() in ext4_jbd2.h tests to see if the
journal is not present before returning true.

Reported-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>