backup verify: do not check every loop iteration for abort/shutdown

only check every 1024'th, which is cheaper to do than a modulo, as we
can just mask the 10 least-significant-bits and check if the result
is zero.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/backup/verify.rs

@@ -185,8 +185,10 @@ fn verify_index_chunks(
     use std::os::unix::fs::MetadataExt;
 
     for pos in 0..index_count {
+        if pos & 1023 == 0 {
             verify_worker.worker.check_abort()?;
             crate::tools::fail_on_shutdown()?;
+        }
 
         let info = index.chunk_info(pos).unwrap();