jbd2: don't write superblock when if its empty
jbd2: don't write superblock when if its empty

This sequence:

# truncate --size=1g fsfile
# mkfs.ext4 -F fsfile
# mount -o loop,ro fsfile /mnt
# umount /mnt
# dmesg | tail

results in an IO error when unmounting the RO filesystem:

[  318.020828] Buffer I/O error on device loop1, logical block 196608
[  318.027024] lost page write due to I/O error on loop1
[  318.032088] JBD2: Error -5 detected when updating journal superblock for loop1-8.

This was a regression introduced by commit 24bcc89c7e7c: "jbd2: split
updating of journal superblock and marking journal empty".

Signed-off-by: Eric Sandeen <d55521028c14e0f378be5444e4ea388161c0d7e7@redhat.com>
Signed-off-by: "Theodore Ts'o" <4ed386e0495d3e109932df055831d9ec2f824927@mit.edu>
Cc: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs