netfilter: nf_tables: remove annotation to access set timeout while holding lock

[ Upstream commit 15d8605c ]

Mutex is held when adding an element, no need for READ_ONCE, remove it.

Fixes: 123b9961 ("netfilter: nf_tables: honor set timeout and garbage collection updates")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>