RDMA/irdma: Fix building without IPv6

RDMA/irdma: Fix building without IPv6

The new irdma_iw_get_vlan_prio() function requires IPv6 support to build:

x86_64-linux-ld: drivers/infiniband/hw/irdma/cm.o: in function `irdma_iw_get_vlan_prio':
cm.c:(.text+0x2832): undefined reference to `ipv6_chk_addr'

Add a compile-time check in the same way as elsewhere in this file to avoid
this by conditionally leaving out the ipv6 specific bits.

Fixes: f877f22ac1e9b ("RDMA/irdma: Implement egress VLAN priority")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230718193835.3546684-1-arnd@kernel.org
Acked-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>