vfio/cdx: prevent read-only region mappings from becoming writable

vfio_cdx_mmap() rejects writable mappings of regions without the WRITE
flag, but leaves VM_MAYWRITE set.  Userspace can map such a region
read-only and then upgrade the mapping to writable with mprotect(),
writing to MMIO regions the device marks read-only.

Clear VM_MAYWRITE for regions without the WRITE flag, as i915 does for
its read-only objects and as fixed in drm/vc4 (CVE-2026-68445),
drm/panthor (CVE-2024-53071) and commit a5edadbae57e ("ptp: vmclock:
prevent read-only mappings from becoming writable").

Note this is defensive hardening: no in-tree cdx device currently
publishes a region without the WRITE flag.  The guard costs nothing and
keeps the mmap() interface honest if a read-only region ever appears.

Fixes: 234489ac56130 ("vfio/cdx: add support for CDX bus")
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
Link: https://lore.kernel.org/r/20260929094848.7439-4-suruurism@gmail.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

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