mm/mempolicy.c: remove unnecessary is_valid_nodemask()

When nodes is true, nsc->mask2 has already been filtered by nsc->mask1,
which has already factored in node_states[N_MEMORY].

Signed-off-by: Zhihui Zhang <zzhsuny@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>