ksmbd: fix missing RDMA-capable flag for IPoIB device in ksmbd_rdma_capable_netdev()

Physical ib_device does not have an underlying net_device, thus its
association with IPoIB net_device cannot be retrieved via
ops.get_netdev() or ib_device_get_by_netdev(). ksmbd reads physical
ib_device port GUID from the lower 16 bytes of the hardware addresses on
IPoIB net_device and match its underlying ib_device using ib_find_gid()

Signed-off-by: Kangjing Huang <huangkangjing@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Steve French <stfrench@microsoft.com>