ksmbd: remove unneeded NULL check in for_each_netdev

netdev can never be NULL in for_each_netdev loop.
This patch remove unneeded NULL check.

Reported-by: Coverity Scan <scan-admin@coverity.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>