xfs_repair: Document & robustify blkmap_next_off()

xfs_repair: Document & robustify blkmap_next_off()

blkmap_next_off() was cryptic (to me), so document what it does.
Also catch cases when the passed in extent index 't' is beyond
the number of extents in the blkmap, so that:

        ext = blkmap->exts + *t;

doesn't walk off the end of the array into garbage.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>