mm/vmscan: pass NULL to trace vmscan node reclaim

mm/vmscan: pass NULL to trace vmscan node reclaim

The tracepoint for node relcaims takes a `struct mem_cgroup *`
as the third argument, so pass NULL instead of 0 to fix warning
about using an integer as a pointer.

Fixes the following warnings:

mm/vmscan.c:6753:66: warning: Using plain integer as NULL pointer
mm/vmscan.c:6757:58: warning: Using plain integer as NULL pointer
mm/vmscan.c:7818:60: warning: Using plain integer as NULL pointer

Link: https://lore.kernel.org/20260616095906.210016-1-ben.dooks@codethink.co.uk
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>