xfs: do not use unchecked extent indices in xfs_bmap_add_extent_*

Make sure to only call xfs_iext_get_ext after we've validate the
extent index in the various xfs_bmap_add_extent_* helpers.

Based on an earlier patch from Lachlan McIlroy.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lachlan McIlroy <lmcilroy@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>