rds_tcp: close NULL deref window in rds_tcp_set_callbacks

rds_tcp: close NULL deref window in rds_tcp_set_callbacks

rds_tcp_set_callbacks() links a new rds_tcp_connection onto
rds_tcp_tc_list under rds_tcp_tc_list_lock. It releases the
lock, then assigns tc->t_sock = sock outside the lock.

rds_tcp_tc_info() and rds6_tcp_tc_info() walk rds_tcp_tc_list
under the same lock. Both dereference tc->t_sock->sk without
a NULL check.

A reader can acquire rds_tcp_tc_list_lock between the writer's
spin_unlock and the t_sock store. It then sees a list entry
whose t_sock is NULL. The dereference of tc->t_sock->sk is a
NULL access.

Move tc->t_sock = sock inside rds_tcp_tc_list_lock, before
list_add_tail. A reader holding the lock then observes the
linkage and the t_sock store together.

The restore path is safe. rds_tcp_restore_callbacks() does
list_del_init inside the lock. The matching tc->t_sock = NULL
after unlink is harmless to readers holding the lock.

Fixes: 70041088e3b9 ("RDS: Add TCP transport to RDS")
Suggested-by: Simon Horman <horms@kernel.org>
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
Reviewed-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260512142807.1855619-1-maoyi.xie@ntu.edu.sg
Signed-off-by: Jakub Kicinski <kuba@kernel.org>