CVE-2024-43832 s390/uv: Don't call folio_wait_writeback() without a folio reference

In the Linux kernel, the following vulnerability has been resolved:

s390/uv: Don’t call folio_wait_writeback() without a folio reference

folio_wait_writeback() requires that no spinlocks are held and that
a folio reference is held, as documented. After we dropped the PTL, the
folio could get freed concurrently. So grab a temporary reference.