netfs: clear PG_private_2 on copy-to-cache append failure

netfs: clear PG_private_2 on copy-to-cache append failure

netfs_pgpriv2_copy_to_cache() marks the folio with PG_private_2 before
netfs_pgpriv2_copy_folio() appends it to the copy-to-cache rolling
buffer.

If the append fails, the folio is not queued for cache writeback, so
the PG_private_2 state and its reference must be released immediately.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Signed-off-by: Yichong Chen <chenyichong@uniontech.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260727130716.1099906-2-dhowells@redhat.com
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>