inet: Remove rtnl_is_held arg of lwtunnel_valid_encap_type(_attr)?().

inet: Remove rtnl_is_held arg of lwtunnel_valid_encap_type(_attr)?().

Commit f130a0cc1b4f ("inet: fix lwtunnel_valid_encap_type() lock
imbalance") added the rtnl_is_held argument as a temporary fix while
I'm converting nexthop and IPv6 routing table to per-netns RTNL or RCU.

Now all callers of lwtunnel_valid_encap_type() do not hold RTNL.

Let's remove the argument.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250516022759.44392-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>