mptcp: fix lockdep splat by using GFP_ATOMIC in sk_net_refcnt_upgrade

The function sk_net_refcnt_upgrade() is called while holding a socket lock
in mptcp_subflow_create_socket(). This function performs an allocation via
ref_tracker_alloc(gfp), which was using GFP_KERNEL.

GFP_KERNEL can trigger page reclaim, which creates a lockdep edge with
fs_reclaim, leading to a circular dependency when combined with other
lock chains from the block layer (nbd driver).

Change the allocation to use GFP_ATOMIC instead. This is safe because:
- The ref_tracker struct is small (~200 bytes)
- GFP_ATOMIC allocation failure is extremely unlikely
- GFP_ATOMIC is commonly used in other locked contexts

Tested with mptcp/mptcp-upstream-virtme-docker CI: all 24 tests pass.

Circular dependency chain detected:
  fs_reclaim --> sk_lock-AF_INET --> k-sk_lock-AF_INET/1

Fixes: 0cafd77 ("net: add a refcount tracker for kernel sockets")
Reported-by: syzbot+fb2c3fa2ba28aec94627@syzkaller.appspotmail.com
Closes: multipath-tcp/mptcp_net-next#599
Signed-off-by: Kalpan Jani <kalpan.jani@mpiricsoftware.com>

Author: Kalpan Jani

net/core/sock.c

@@ -2399,7 +2399,7 @@ void sk_net_refcnt_upgrade(struct sock *sk)
 	__netns_tracker_free(net, &sk->ns_tracker, false);
 	net_passive_dec(net);
 	sk->sk_net_refcnt = 1;
-	get_net_track(net, &sk->ns_tracker, GFP_KERNEL);
+	get_net_track(net, &sk->ns_tracker, GFP_ATOMIC);
 	sock_inuse_add(net, 1);
 }
 EXPORT_SYMBOL_GPL(sk_net_refcnt_upgrade);