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>