gbmap: Renamed gbmap_scan_thresh -> gbmap_rebuild_thresh

I think a good rule of thumb is if you refer to some variable/config/
field with a different name in comments/writing/etc more often than not,
you should just rename the variable/config/field to match.

So yeah, gbmap_rebuild_thresh controls when the gbmap is rebuilt.

Also touched up the doc comment a bit.

lfs3.c

@@ -10973,7 +10973,7 @@ static inline int lfs3_alloc_ckpoint(lfs3_t *lfs3) {
     // do we need to rebuild the gbmap?
     if (lfs3_f_isgbmap(lfs3->flags)
             && lfs3->lookahead.gbmapped < lfs3_min(
-                lfs3->cfg->gbmap_scan_thresh,
+                lfs3->cfg->gbmap_rebuild_thresh,
                 lfs3->block_count)) {
         int err = lfs3_alloc_rebuildgbmap(lfs3);
         if (err) {