xfs: regularize iclog space accounting in xlog_write_partial

xfs: regularize iclog space accounting in xlog_write_partial

When xlog_write_partial splits a log region over multiple iclogs, it
has to include the continuation ophder in the length requested for the
new iclog.  Currently is simply adds that to the request, which makes
the accounting of the used space below look slightly different from the
other users of iclog space that decrement it.

To prepare for more code sharing, add the ophdr size to the len variable
that tracks the number of bytes still are left in this xlog_write
operation before the calling xlog_write_get_more_iclog_space, and then
decrement it later when consuming that space.

This changes the value of len when xlog_write_get_more_iclog_space
returns an error, but as nothing looks at len in that case the
difference doesn't matter.

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