MDEV-15779 - mariabackup incremental prepare fails on CIFS mount.

CIFS does not like O_DIRECT flag (it is set successfully, but pread would
fail).

The fix is not to use O_DIRECT, there is not need for it.
posix_fadvise() was used already that should prevent buffer cache
pollution on Linux.

As recommended by documentation of posix_fadvise(), we'll also fsync()
tablespaces after a batch of writes.

extra/mariabackup/xtrabackup.cc

@@ -4966,8 +4966,6 @@ xtrabackup_apply_delta(
 
 	posix_fadvise(src_file, 0, 0, POSIX_FADV_SEQUENTIAL);
 
-	os_file_set_nocache(src_file, src_path, "OPEN");
-
 	dst_file = xb_delta_open_matching_space(
 			dbname, space_name, info.space_id, info.zip_size,
 			dst_path, sizeof(dst_path), &success);
@@ -4978,8 +4976,6 @@ xtrabackup_apply_delta(
 
 	posix_fadvise(dst_file, 0, 0, POSIX_FADV_DONTNEED);
 
-	os_file_set_nocache(dst_file, dst_path, "OPEN");
-
 	/* allocate buffer for incremental backup (4096 pages) */
 	incremental_buffer_base = static_cast<byte *>
 		(ut_malloc((page_size / 4 + 1) *
@@ -5079,6 +5075,13 @@ xtrabackup_apply_delta(
 			}
 		}
 
+		/* Free file system buffer cache after the batch was written. */
+#ifdef __linux__
+		os_file_flush_func(dst_file);
+#endif
+		posix_fadvise(dst_file, 0, 0, POSIX_FADV_DONTNEED);
+
+
 		incremental_buffers++;
 	}