libpager, ext2fs: add bulk page write support

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)

0 files changed, 0 insertions, 0 deletions