net: fix sock compilation error under CONFIG_PREEMPT_RT

net: fix sock compilation error under CONFIG_PREEMPT_RT

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>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228111319.79506-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>