xfs: remove the extra buffer reference in xfs_buf_submit

Nothing touches the buffer after it has been submitted now, so the need for
the extra transient reference went away as well.

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>