mm: multi-gen LRU: remove NULL checks on NODE_DATA()

NODE_DATA() is preallocated for all possible nodes after commit
09f49dca ("mm: handle uninitialized numa nodes gracefully").  Checking
its return value against NULL is now unnecessary.

Link: https://lkml.kernel.org/r/20221116013808.3995280-2-yuzhao@google.com


Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>