KVM: selftests: Account for kernel's off-by-one bug in NUMA node syscalls

Add and use MAXNODE_FOR_MASK() to compute the "correct" maxnode value that
is passed to various NUMA-related syscalls, e.g. get_mempolicy(), mbind(),
migrate_pages(), etc.  In quotes, because the kernel has an undocumented,
longstanding off-by-one bug that requires userspace to specify the number
of bits plus one, i.e. the max node plus two.

The kernel bug has been known since 2007[*]:

 : And this is I think the reason why we can't change this now. I assume
 : numactl allocates 1024 bits (0 to 1023) and passes 1025 to make sure all
 : 1024 bits are processed. If we change it now, kernel will process 1025
 : bits (0 to 1024) and overflow the allocated bitmask. If it happens to be
 : at the border of mmaped vma, it's a segfault...

But unfortunately the manpages haven't yet been updated, e.g.

 : The maxnode argument is the maximum node number in the bit mask plus one

Link: https://lore.kernel.org/all/63ccc890-fd57-118b-5997-e0259f507d28@suse.cz[*]
Reviewed-by: Shivank Garg <shivankg@amd.com>
Link: https://patch.msgid.link/20260903001625.2792367-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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