xfs: don't use XFS_BMAPI_ENTRIRE in xfs_get_blocks

There is no reason to get a mapping bigger than what we were asked for.

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>