RDMA/rxe: Restore HMM_PFN_WRITE check in ODP write paths

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

RDMA/rxe: Restore HMM_PFN_WRITE check in ODP write paths

Commit 0b261d7c1cd3 ("RDMA/rxe: Break endless pagefault loop for RO
pages") dropped the access permission test from rxe_check_pagefault()
and left only HMM_PFN_VALID. A page faulted in read-only, for example
a page-cache folio behind a PROT_READ file mapping, then satisfies the
check and ODP write operations (RDMA WRITE, RDMA READ response, SEND
payload, atomics) modify it through kmap without ever breaking CoW.

An unprivileged user can register an ODP MR over such a mapping and
have incoming RDMA traffic overwrite the page cache of a file it only
holds O_RDONLY, including /etc/passwd or setuid binaries. This is the
same primitive class as Dirty COW and CVE-2022-2590.

mlx5 has the missing invariant: its ODP path sets the device write bit
only for pfns that carry HMM_PFN_WRITE. Restore it in rxe by requiring
HMM_PFN_WRITE in rxe_check_pagefault() for every operation except
RXE_PAGEFAULT_RDONLY. A write to a non-writable VMA now fails the one
fault attempt with -EPERM from hmm_vma_fault() instead of re-faulting
forever. For a writable VMA the fault breaks CoW and the write lands
in the private page.

Keep pmem flushes on the read-only check. arch_wb_cache_pmem() never
modifies memory, and the FLUSH access bits do not make the umem
writable, so classifying flushes as writes would make every flush
against a flush-only MR fail.