refactor: Add AutoFile::size

src/wallet/migrate.cpp

@@ -544,8 +544,7 @@ void BerkeleyRODatabase::Open()
     page_size = outer_meta.pagesize;
 
     // Verify the size of the file is a multiple of the page size
-    db_file.seek(0, SEEK_END);
-    int64_t size = db_file.tell();
+    const int64_t size{db_file.size()};
 
     // Since BDB stores everything in a page, the file size should be a multiple of the page size;
     // However, BDB doesn't actually check that this is the case, and enforcing this check results