xfs: improve the ->iop_format interface

xfs: improve the ->iop_format interface

Export a higher level interface to format log items.  The xlog_format_buf
structure is hidden inside xfs_log_cil.c and only accessed using two
helpers (and a wrapper build on top), hiding details of log iovecs from
the log items.  This also allows simply using an index into lv_iovecp
instead of keeping a cursor vec.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>