basebackup: use Timeline::get for get_rel instead of get_rel_page_at_lsn (#10476)

I noticed the opportunity to simplify here while working on
https://github.com/neondatabase/neon/pull/9353 .

The only difference is the zero-fill behavior: if one reads past rel
size,
`get_rel_page_at_lsn` returns a zeroed page whereas `Timeline::get`
returns an error.

However, the `endblk` is at most rel size large, because `nblocks` is eq
`get_rel_size`, see a few lines above this change.

We're using the same LSN (`self.lsn`) for everything, so there is no
chance of non-determinism.

Refs:

- Slack discussion debating correctness:
https://neondb.slack.com/archives/C033RQ5SPDH/p1737457010607119

pageserver/src/basebackup.rs

@@ -13,7 +13,7 @@
 use anyhow::{anyhow, Context};
 use bytes::{BufMut, Bytes, BytesMut};
 use fail::fail_point;
-use pageserver_api::key::Key;
+use pageserver_api::key::{rel_block_to_key, Key};
 use postgres_ffi::pg_constants;
 use std::fmt::Write as FmtWrite;
 use std::time::{Instant, SystemTime};
@@ -501,13 +501,9 @@ where
             for blknum in startblk..endblk {
                 let img = self
                     .timeline
-                    .get_rel_page_at_lsn(
-                        src,
-                        blknum,
-                        Version::Lsn(self.lsn),
-                        self.ctx,
-                        self.io_concurrency.clone(),
-                    )
+                    // TODO: investigate using get_vectored for the entire startblk..endblk range.
+                    // But this code path is not on the critical path for most basebackups (?).
+                    .get(rel_block_to_key(src, blknum), self.lsn, self.ctx)
                     .await
                     .map_err(|e| BasebackupError::Server(e.into()))?;
                 segment_data.extend_from_slice(&img[..]);