xfs: ensure correct log item buffer alignment

On 32 bit platforms, the log item vector headers are not 64 bit
aligned or sized. hence if we don't take care to align them
correctly or pad the buffer appropriately for 8 byte alignment, we
can end up with alignment issues when accessing the user buffer
directly as a structure.

To solve this, simply pad the buffer headers to 64 bit offset so
that the data section is always 8 byte aligned.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reported-by: Michael L. Semon <mlsemon35@gmail.com>
Tested-by: Michael L. Semon <mlsemon35@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>