xfs: remove prev argument to xfs_bmapi_reserve_delalloc

We can easily lookup the previous extent for the cases where we need it,
which saves the callers from looking it up for us later in the series.

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