RFC: adding a crc field to xfs_buf_log_format_t

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

With adding CRC to xfs metadata structures we face an interesting
problem.  As we want all the CRCs logged we always have to log the CRC.
But this means for many modification that previous just required one
or two chunks of a buffer logged (e.g. superblock counters or changing a
single btree records) we would now need a second iovec.  One idea to
avoid this would be to put the crc into a modified version of
xfs_buf_log_format_t.  Any opinions wether this is worth the trouble?