xfs: improve the calling convention for the xlog_write helpers

xfs: improve the calling convention for the xlog_write helpers

The xlog_write chain passes around the same seven variables that are
often passed by reference. Add a xlog_write_data structure to contain
them to improve code generation and readability.

This change increases the generated code size by about 140 bytes for my
x86_64 build, which is hopefully worth the much easier to follow code:

$ size fs/xfs/xfs_log.o*
   text	   data	    bss	    dec	    hex	filename
  29300	   1730	    176	  31206	   79e6	fs/xfs/xfs_log.o
  29160	   1730	    176	  31066	   795a	fs/xfs/xfs_log.o.old

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