mm/ksm.c: make stable_node_dup() static

stable_node_dup() is local to the source and does not need to be in
global scope, so make it static.

Cleans up sparse warning:

  mm/ksm.c:1321:13: warning: symbol 'stable_node_dup' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20180206221005.12642-1-colin.king@canonical.com
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>