x86/mm/pat: Fix effective RW computation in lookup_address_in_pgd_attr()

lookup_address_in_pgd_attr() accumulates the effective NX and RW bits of
the walked page table levels so that verify_rwx() can detect mappings that
are both writable and executable.

The RW bits are folded into a bool with

	rw &= pXd_flags(*pXd) & _PAGE_RW;

but _PAGE_RW is 0x2. So consider the accumulation line:

        rw &= pXd_flags(*pXd) & _PAGE_RW;

where rw=0x1 and the right side evaluates down to 0x2. It'll end up doing:

        rw = 0x1 & 0x2

and rw always ends up 0.

This way rw becomes false at the first level walked, regardless of the
actual permissions, and verify_rwx() treats every mapping as non-writable
and never reports a W^X violation.

Add double negation to the right side to normalize the _PAGE_RW flag to
0 or 1.

Assisted-by: Copilot:claude-opus-4.8
Fixes: ceb647b4b529 ("x86/pat: Introduce lookup_address_in_pgd_attr()")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Tested-by: syzbot@syzkaller.appspotmail.com
Tested-by: Atish Patra <atishp@meta.com>
Tested-by: Nikunj A Dadhania <nikunj@amd.com>
Cc:stable@vger.kernel.org
Link: https://patch.msgid.link/20260813-cpa-fixes-v2-5-39b4ff90f91d@kernel.org

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