mm: memcg: fix NULL pointer in mem_cgroup_track_foreign_dirty_slowpath()

In the Linux kernel, the following vulnerability has been resolved:

mm: memcg: fix NULL pointer in mem_cgroup_track_foreign_dirty_slowpath()

As commit 18365225f044 ("hwpoison, memcg: forcibly uncharge LRU pages"),

hwpoison will forcibly uncharg a LRU hwpoisoned page, the folio_memcg

could be NULl, then, mem_cgroup_track_foreign_dirty_slowpath() could

occurs a NULL pointer dereference, let's do not record the foreign

writebacks for folio memcg is null in mem_cgroup_track_foreign_dirty() to

fix it.