arch/x86: replace nodes_weight with nodes_empty where appropriate

mm code calls nodes_weight() to check if any bit of a given nodemask is
set. We can do it more efficiently with nodes_empty() because nodes_empty()
stops traversing the nodemask as soon as it finds first set bit, while
nodes_weight() counts all bits unconditionally.

Signed-off-by: Yury Norov <yury.norov@gmail.com>