xfs: merge xfs_bmap_read_extents into xfs_iread_extents

xfs_iread_extents is just a trivial wrapper, there is no good reason
to keep the two separate.

[darrick: minor fixups having left xfs_bmbt_validate_extent intact]

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>