xfs: correctly decrement the extent buffer index in xfs_bmap_del_extent

The code in xfs_bmap_del_extent does not correctly decrement the
extent buffer index when deleting a whole extent.  Most of the time
this gets caught by checks in xfs_bmapi that work around it and
decrement it manually and thus wasn't noticed so far.

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>