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.