net, proc: Provide PROC_FS=n fallback for proc_create_net_single_write()

[ Upstream commit c3d96f690a790074b508fe183a41e36a00cd7ddd ]

Provide a CONFIG_PROC_FS=n fallback for proc_create_net_single_write().

Also provide a fallback for proc_create_net_data_write().

Fixes: 564def71 ("proc: Add a way to make network proc files writable")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>