xfs: do not use unchecked extent indices in xfs_bunmapi

Make sure to only call xfs_iext_get_ext after we've validate the
extent index when moving on to the next index in xfs_bunmapi.  Also
remove the old workaround for too large indices that has been
superceeded by the proper fix in xfs_bmap_del_extent.

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>

diff --cc fs/xfs/xfs_bmap.c
Simple merge