mm/memory.c:zap_pte_range() print bad swap entry

We have a report of this WARN() triggering.  Let's print the offending
swp_entry_t to help diagnosis.

Link: https://lkml.kernel.org/r/000000000000b0e576060a30ee3b@google.com
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -1517,6 +1517,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 				continue;
 		} else {
 			/* We should have covered all the swap entry types */
+			pr_alert("unrecognized swap entry 0x%lx\n", entry.val);
 			WARN_ON_ONCE(1);
 		}
 		pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);