Ext4 does not support data journalling with delayed allocation enabled.
We even do not allow to mount the file system with delayed allocation
and data journalling enabled, however it can be set via FS_IOC_SETFLAGS
so we can hit the inode with EXT4_INODE_JOURNAL_DATA set even on file
system mounted with delayed allocation (default) and that's where
problem arises. The easies way to reproduce this problem is with the
following set of commands:

 mkfs.ext4 /dev/sdd
 mount /dev/sdd /mnt/test1
 dd if=/dev/zero of=/mnt/test1/file bs=1M count=4
 chattr +j /mnt/test1/file
 dd if=/dev/zero of=/mnt/test1/file bs=1M count=4 conv=notrunc
 chattr -j /mnt/test1/file

Additionally it can be reproduced quite reliably with xfstests 272 and
269. In fact the above reproducer is a part of test 272.

To fix this we should ignore the EXT4_INODE_JOURNAL_DATA inode flag if
the file system is mounted with delayed allocation. This can be easily
done by fixing ext4_should_*_data() functions do ignore data journal
flag when delalloc is set (suggested by Ted). We also have to set the
appropriate address space operations for the inode (again, ignoring data
journal flag if delalloc enabled).

Additionally this commit introduces ext4_inode_journal_mode() function
because ext4_should_*_data() has already had a lot of common code and
this change is putting it all into one function so it is easier to
read.

Successfully tested with xfstests in following configurations:

delalloc + data=ordered
delalloc + data=writeback
data=journal
nodelalloc + data=ordered
nodelalloc + data=writeback
nodelalloc + data=journal
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org