xfs: introduce xlog_copy_iovec

Add a helper to abstract out filling the log iovecs in the log item
format handlers.  This will allow us to change the way we do the log
item formatting more easily.

The copy in the name is a bit confusing for now as it just assigns a
pointer and lets the CIL code perform the copy, but that will change
soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>