pagebench: fixup after is_rel_block_key changes in #6266 (#6303)

PR #6266 broke the getpage_latest_lsn benchmark.

Before this patch, we'd fail with

```
not implemented: split up range
```

because `r.start = rel size key` and `r.end = rel size key + 1`.

The filtering of the key ranges in that loop is a bit ugly, but,
I measured:
* setup with 180k layer files (20k tenants * 9 layers).
* total physical size is 463GiB
* 5k tenants, the range filtering takes `0.6 seconds` on an
i3en.3xlarge.
That's a tiny fraction of the overall time it takes for pagebench to get
ready to send requests. So, this is good enough for now / there are
other bottlenecks that are bigger.

libs/pageserver_api/src/key.rs

@@ -141,6 +141,7 @@ impl Key {
     }
 }
 
+#[inline(always)]
 pub fn is_rel_block_key(key: &Key) -> bool {
     key.field1 == 0x00 && key.field4 != 0 && key.field6 != 0xffffffff
 }