net: don't call update_pmtu unconditionally

commit f15ca723c1ebe6c1a06bc95fda6b62cd87b44559 upstream.

Some dst_ops (e.g. md_dst_ops)) doesn't set this handler. It may result to:
"BUG: unable to handle kernel NULL pointer dereference at           (null)"

Let's add a helper to check if update_pmtu is available before calling it.

Fixes: 52a589d51f10 ("geneve: update skb dst pmtu on tx path")
Fixes: a93bf0ff4490 ("vxlan: update skb dst pmtu on tx path")
CC: Roman Kapl <code@rkapl.cz>
CC: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Thomas Deutschmann <whissi@gentoo.org>
Cc: Eddie Chapman <eddie@ehuk.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>