proc.5: Add "wf" to VmFlags in /proc/[pid]/smaps

proc.5: Add "wf" to VmFlags in /proc/[pid]/smaps

This patch documents a flag added in the following kernel commit:

commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
Author: Rik van Riel <riel@redhat.com>
Date:   Wed Sep 6 16:25:15 2017 -0700

    mm,fork: introduce MADV_WIPEONFORK

This was already documented in man2/madvise.2 in the commit:

commit c0c4f6c29c494c466f3a2a6273c5b55b76a72927
Author: Rik van Riel <riel@redhat.com>
Date:   Tue Sep 19 20:32:00 2017 +0200

    madvise.2: Document MADV_WIPEONFORK and MADV_KEEPONFORK

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>