mm/mempolicy.c: fix mempolicy printing in numa_maps

mm/mempolicy.c: fix mempolicy printing in numa_maps

As a result of commit 5606e3877ad8 ("mm: numa: Migrate on reference
policy"), /proc/<pid>/numa_maps prints the mempolicy for any <pid> as
"prefer:N" for the local node, N, of the process reading the file.

This should only be printed when the mempolicy of <pid> is
MPOL_PREFERRED for node N.

If the process is actually only using the default mempolicy for local
node allocation, make sure "default" is printed as expected.

Signed-off-by: David Rientjes <[email protected]>
Reported-by: Robert Lippert <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: <[email protected]>	[3.7+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>