Add local mm_rss_stat definition

Summary:
Add local definition of struct mm_rss_stat available pre-6.2. Once we switch to
6.4-based vmlinux.h in fbcode this stop compiling because `struct mm_rss_stat`
is not available anymore.

Reviewed By: lnyng

Differential Revision: D56500799

fbshipit-source-id: e5371bae807615011d6f615ea78ea5e7bd83b9ab

below/src/bpf/exitstat.bpf.c

@@ -68,8 +68,12 @@ struct task_struct___post516 {
   struct thread_info___post516 thread_info;
 } __attribute__((preserve_access_index));
 
+struct mm_rss_stat___pre62 {
+	atomic_long_t count[4];
+} __attribute__((preserve_access_index));
+
 struct mm_struct___pre62 {
-	struct mm_rss_stat rss_stat;
+	struct mm_rss_stat___pre62 rss_stat;
 } __attribute__((preserve_access_index));
 
 struct mm_struct___post62 {