Disable creation of a UDP socket for ipv6 when
CONFIG_IPV6 is not enabeld. Since udp_sock_create6()
returns 0 when CONFIG_IPV6 is not set

[   46.888632] IP: [<c220705a>] setup_udp_tunnel_sock+0x6/0x4f
[   46.891355] *pdpt = 0000000000000000 *pde = f000ff53f000ff53
[   46.893918] Oops: 0002 [#1] PREEMPT
[   46.896014] CPU: 0 PID: 1 Comm: swapper Not tainted 4.7.0-rc4-00001-g8700e3e7 #1
[   46.900280] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Debian-1.8.2-1 04/01/2014
[   46.904905] task: cf06c040 ti: cf05e000 task.ti: cf05e000
[   46.907854] EIP: 0060:[<c220705a>] EFLAGS: 00210246 CPU: 0
[   46.911137] EIP is at setup_udp_tunnel_sock+0x6/0x4f
[   46.914070] EAX: 00000044 EBX: 00000001 ECX: cf05fef0 EDX: ca8142e0
[   46.917236] ESI: c2c4505b EDI: cf05fef0 EBP: cf05fed0 ESP: cf05fed0
[   46.919836]  DS: 007b ES: 007b FS: 0000 GS: 00e0 SS: 0068
[   46.922046] CR0: 80050033 CR2: 000001fc CR3: 02cec000 CR4: 000006b0
[   46.924550] Stack:
[   46.926014]  cf05ff10 c1fd4657 ca8142e0 0000000a 00000000 00000000 0000b712 00000008
[   46.931274]  00000000 6bb5bd01 c1fd48de 00000000 00000000 cf05ff1c 00000000 00000000
[   46.936122]  cf05ff1c c1fd4bdf 00000000 cf05ff28 c2c4507b ffffffff cf05ff88 c2bf1c74
[   46.942350] Call Trace:
[   46.944403]  [<c1fd4657>] rxe_setup_udp_tunnel+0x8f/0x99
[   46.947689]  [<c1fd48de>] ? net_to_rxe+0x4e/0x4e
[   46.950567]  [<c1fd4bdf>] rxe_net_init+0xe/0xa4
[   46.953147]  [<c2c4507b>] rxe_module_init+0x20/0x4c
[   46.955448]  [<c2bf1c74>] do_one_initcall+0x89/0x113
[   46.957797]  [<c2bf15eb>] ? set_debug_rodata+0xf/0xf
[   46.959966]  [<c2bf1dbc>] ? kernel_init_freeable+0xbe/0x15b
[   46.962262]  [<c2bf1ddc>] kernel_init_freeable+0xde/0x15b
[   46.964418]  [<c232eb54>] kernel_init+0x8/0xd0
[   46.966618]  [<c2333122>] ret_from_kernel_thread+0xe/0x24
[   46.969592]  [<c232eb4c>] ? rest_init+0x6f/0x6f

Fixes: 8700e3e7 ("Soft RoCE driver")
Signed-off-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>