xfs: clean up log buffer writes

Don't bother using XFS_bwrite as it doesn't provide much code for
our use case.  Instead opencode it and fold xlog_bdstrat_cb into the
new xlog_bdstrat helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>