xfs: remove bmap block allocation retries

Now that reflink operations don't set the firstblock value we don't
need the workarounds for non-NULL firstblock values without a prior
allocation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>