mm/memfd_luo: report error when restoring a folio fails mid-loop

mm/memfd_luo: report error when restoring a folio fails mid-loop

memfd_luo_retrieve_folios() initialises err to -EIO, but the per-iteration
calls to mem_cgroup_charge(), shmem_add_to_page_cache() and
shmem_inode_acct_blocks() reuse and overwrite err.  Once any iteration
completes successfully, err becomes zero.

If a later iteration's kho_restore_folio() returns NULL, the failure path
jumps to put_folios without resetting err, so the function returns 0.
The caller memfd_luo_retrieve() then takes the success path, sets
args->file and reports the restore as successful, leaving userspace with
a partially populated memfd and no indication that anything went wrong.

Set err to -EIO in the kho_restore_folio() failure branch so the error
is propagated to the caller.

Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
Link: https://patch.msgid.link/20260415052300.362539-1-devnexen@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>