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Fix the default behaviour for the remap operators in bitmap, cpumask and
nodemask.

As previously submitted, the pair of masks <A, B> defined a map of the
positions of the set bits in A to the corresponding bits in B.  This is still
true.

The issue is how to map the other positions, corresponding to the unset (0)
bits in A.  As previously submitted, they were all mapped to the first set bit
position in B, a constant map.

When I tried to code per-vma mempolicy rebinding using these remap operators,
I realized this was wrong.

This patch changes the default to map all the unset bit positions in A to the
same positions in B, the identity map.

For example, if A has bits 4-7 set, and B has bits 9-12 set, then the map
defined by the pair <A, B> maps each bit position in the first 32 bits as
follows:

	0 ==> 0
	  ...
	3 ==> 3
	4 ==> 9
	  ...
	7 ==> 12
	8 ==> 8
	9 ==> 9
	  ...
	31 ==> 31

This now corresponds to the typical behaviour desired when migrating pages and...