Merge patch series "netfs: Miscellaneous fixes"

David Howells <dhowells@redhat.com> says:

Here are some miscellaneous fixes for netfslib.

 (1) Clear PG_private_2 on copy-to-cache append failure.

 (2) Fix handling of rolling buffer allocation failure in single-object
     writeback.  This is probably unnecessary with (4), but if we're only
     writing to the cache, we can skip the write.

 (3) Fix cleanup of readeahead folios if iterator preparation fails.

 (4) Fix folio_queue allocation failure in writeback by adding a mempool.
     This also improves request and subrequest allocation.

* patches from https://patch.msgid.link/20260727130716.1099906-1-dhowells@redhat.com:
  netfs: Fix folio_queue ENOMEM in writeback by adding a mempool
  netfs: release readahead folios on iterator preparation failure
  netfs: handle single writeback rolling buffer allocation failure
  netfs: clear PG_private_2 on copy-to-cache append failure

Link: https://patch.msgid.link/20260727130716.1099906-1-dhowells@redhat.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>