Revert "ipv4: use RCU protection in __ip_rt_update_pmtu()"

This reverts commit ce3c6165 which is
commit 13951219 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Iaa60ee49e8763010c46ecbd93d95d1eecc17cd9b
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>