Refactor fellow_busy_region_free()

As the fdr argument is from fbo->region, we should DTRT for this
situation and not bloat the code with a loop and stuff...

Motivated by #39

src/fellow_cache.c

@@ -2183,28 +2183,26 @@ static void
 fellow_busy_region_free(struct fellow_busy *fbo, struct buddy_off_extent *fdr)
 {
 	struct fellow_cache *fc;
-	unsigned u;
+	ssize_t u;
 
 	CHECK_OBJ_NOTNULL(fbo, FELLOW_BUSY_MAGIC);
 
 	fc = fbo->fc;
 	CHECK_OBJ_NOTNULL(fc, FELLOW_CACHE_MAGIC);
 
-	AN(fbo->nregion);
-	u = fbo->nregion;
-	while (u--) {
-		if (fdr != &fbo->region[u])
-			continue;
-		fellow_region_free(fc->ffd, fdr);
-		fbo->nregion--;
-		assert(u <= fbo->nregion);
-		if (u < fbo->nregion) {
-			memmove(&fbo->region[u], &fbo->region[u] + 1,
-			    (fbo->nregion - u) * sizeof *fdr);
-		}
+	u = fdr - &fbo->region[0];
+	assert(u >= 0);
+	assert(u < fbo->nregion);
+
+	// obviously: assert(&fbo->region[u] == fdr);
+
+	fellow_region_free(fc->ffd, fdr);
+
+	if (u == --fbo->nregion)
 		return;
-	}
-	WRONG("region to free not found");
+
+	memmove(&fbo->region[u], &fbo->region[u + 1],
+	    (fbo->nregion - (size_t)u) * sizeof *fdr);
 }
 
 #ifdef TEST_DRIVER