cleanup log item formatting v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

I dug into a rabit hole about the log item formatting recently,
and noticed that the handling of the opheaders is still pretty
ugly because it leaks pre-delayed logging implementation
details into the log item implementations.

The core of this series is to remove the to reserve space in the
CIL buffers/shadow buffers for the opheaders that already were
generated more or less on the fly by the lowlevel log write
code anyway, but there's lots of other cleanups around it.

Note that sits on top of the "kill xlog_in_core_2_t v3" because
a struct removal there sits right next to a struct moved here.

Changes since v1:
 - rebased and dropped the already merged patches

Diffstat:
 libxfs/xfs_log_format.h |    7 -
 xfs_attr_item.c         |   27 +---
 xfs_bmap_item.c         |   10 -
 xfs_buf_item.c          |   19 +--
 xfs_dquot_item.c        |    9 -
 xfs_exchmaps_item.c     |   11 -
 xfs_extfree_item.c      |   10 -
 xfs_icreate_item.c      |    6 
 xfs_inode_item.c        |   49 +++-----
 xfs_log.c               |  292 ++++++++++++++++++------------------------------
 xfs_log.h               |   65 +---------
 xfs_log_cil.c           |  111 ++++++++++++++++--
 xfs_log_priv.h          |   20 +++
 xfs_refcount_item.c     |   10 -
 xfs_rmap_item.c         |   10 -
 xfs_trans.h             |    4 
 16 files changed, 313 insertions(+), 347 deletions(-)