libpager, ext2fs: add bulk page write supportHEADmaster

Introduce a new pager_write_pages() entry point, allowing filesystems
to implement multi-page writes in one call.  libpager will attempt to
coalesce contiguous dirty pages and call this bulk interface; if it is
unsupported or only partially completes, the remaining pages are
handled by the existing per-page path.

ext2fs now provides file_pager_write_pages(), implemented using a
small chunked write loop (currently 2 blocks per chunk) under
alloc_lock.  This reduces lock/unlock cycles, improves batching of
store_write() calls, and avoids starvation by yielding between chunks.

Other filesystems may continue using pager_write_page() unchanged.

Test write 128 MiB dirty file:
Before this change:
store_write calls: 32,874; avg: 4,096 B

After this change (coalesced runs, cap=128):
store_write calls: 1,300; avg 103 KiB
(~25 times fewer calls, ~25 times larger writes)

1 files changed, 37 insertions, 0 deletions

diff --git a/libpager/pager-bulk.c b/libpager/pager-bulk.c
new file mode 100644
index 00000000..8dba01b5
--- /dev/null
+++ b/libpager/pager-bulk.c
@@ -0,0 +1,37 @@
+/* pager-bulk.c Default (dummy) implementation of bulk page write.
+
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   Written by Milos Nikic.
+
+   This file is part of the GNU Hurd.
+
+   The GNU Hurd is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   The GNU Hurd is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with the GNU Hurd; if not, see <https://www.gnu.org/licenses/>.  */
+
+#include <libpager/pager.h>
+#include "priv.h"
+
+/* Default dummy implementation of pager_write_pages. */
+__attribute__((weak)) error_t
+pager_write_pages (struct user_pager_info *upi,
+		   vm_offset_t offset,
+		   vm_address_t data, vm_size_t length, vm_size_t *written)
+{
+  (void) upi;
+  (void) offset;
+  (void) data;
+  (void) length;
+  if (written)
+    *written = 0;
+  return EOPNOTSUPP;
+}