mempolicy: document {set|get}_policy() vm_ops APIs

Document mempolicy return value reference semantics assumed by the rest of the
mempolicy code for the set_ and get_policy vm_ops in <linux/mm.h>--where the
prototypes are defined--to inform any future mempolicy vm_op writers what the
rest of the subsystem expects of them.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>