ctdb-scripts: Fix tcp_tw_recycle existence check

ctdb-scripts: Fix tcp_tw_recycle existence check

net.ipv4.tcp_tw_recycle has been removed from Linux 4.12 but, still,
makes sense to check its existence. Unfortunately, current check does
not test for the procfs file existence. This commit fixes the issue.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13984

Signed-off-by: Rafael David Tinoco <rafaeldtinoco@ubuntu.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Tue Jun  4 23:31:24 UTC 2019 on sn-devel-184

(cherry picked from commit 843fbb1207ee7ac84f3282974b66b9290d8da0ac)