[PATCH net-next v2] net: fix sock compilation error under CONFIG_PREEMPT_RT

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

When CONFIG_PREEMPT_RT is enabled, __SPIN_LOCK_UNLOCKED() expands to a
brace-enclosed initializer rather than a compound literal, which cannot
be used in assignment expressions. This causes a build failure:

  net/core/sock.c:3787:29: error: expected expression before '{' token
   3787 |                 tmp.slock = __SPIN_LOCK_UNLOCKED(tmp.slock);

Use declaration-with-initializer instead of assignment, consistent with
how __SPIN_LOCK_UNLOCKED() is used elsewhere in the kernel (e.g.
DEFINE_SPINLOCK).

Fixes: 5151ec54f586 ("net: use try_cmpxchg() in lock_sock_nested()")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
 net/core/sock.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 9d841975a7a1..fba4d5b8553c 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3782,12 +3782,15 @@ void noinline lock_sock_nested(struct sock *sk, int subclass)
 	might_sleep();
 #ifdef CONFIG_64BIT
 	if (sizeof(struct slock_owned) == sizeof(long)) {
-		socket_lock_t tmp, old;
+		socket_lock_t tmp = {
+			.slock = __SPIN_LOCK_UNLOCKED(tmp.slock),
+			.owned = 1,
+		};
+		socket_lock_t old = {
+			.slock = __SPIN_LOCK_UNLOCKED(old.slock),
+			.owned = 0,
+		};
 
-		tmp.slock = __SPIN_LOCK_UNLOCKED(tmp.slock);
-		tmp.owned = 1;
-		old.slock = __SPIN_LOCK_UNLOCKED(old.slock);
-		old.owned = 0;
 		if (likely(try_cmpxchg(&sk->sk_lock.combined,
 				       &old.combined, tmp.combined)))
 			return;
-- 
2.43.0