Fix for the "kmem -z" option on Linux 5.0 and later kernels

that contain commit a921444382b49cc7fdeca3fba3e278bc09484a27,
titled "mm: move zone watermark accesses behind an accessor".
Without the patch, the command fails with the error message
"kmem: invalid (optional) structure member offsets: zone_pages_min
or zone_struct_pages_min".
(k-hagio@ab.jp.nec.com)

memory.c

@@ -1095,6 +1095,9 @@ vm_init(void)
                                 "zone", "pages_high");
                         MEMBER_OFFSET_INIT(zone_watermark,
                                 "zone", "watermark");
+                        if (INVALID_MEMBER(zone_watermark))
+                                MEMBER_OFFSET_INIT(zone_watermark,
+                                        "zone", "_watermark");
                         MEMBER_OFFSET_INIT(zone_nr_active,
                                 "zone", "nr_active");
                         MEMBER_OFFSET_INIT(zone_nr_inactive,