fellow_busy: Async allocation for ..._body_seg_alloc()

src/fellow_cache.c

@@ -2508,6 +2508,11 @@ fellow_busy_body_seg_alloc(struct fellow_busy *fbo,
 		fbr->reg = fdr;
 		AZ(fbr->len);
 		fbo->sz_dskalloc += fdr->size;
+
+		// async allocation for remaining known size
+		szc = fellow_busy_body_size_strategy(fbo);
+		if (szc.sz)
+			AN(buddy_req_extent(reqs, szc.sz, szc.cram));
 	}
 
 	assert(fbr->len < fdr->size);