xfs: add a b_iodone callback to struct xfs_buf

Stop open coding the log item completions and instead add a callback
into back into the submitter.

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