net: Remove unnecessary NULL check for lwtunnel_fill_encap()

lwtunnel_fill_encap() has NULL check and return 0, so no need
to check before call it.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250624140015.3929241-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>