rds: tcp: unregister sysctl before tearing down listen socket

rds: tcp: unregister sysctl before tearing down listen socket

rds_tcp_exit_net() frees the per-netns RDS TCP listen socket via
rds_tcp_kill_sock() before unregistering the per-netns sysctl table.  Since
rds_tcp_skbuf_handler() derives the netns from
rtn->rds_tcp_listen_sock->sk, a concurrent sysctl write can race with
netns teardown and dereference the freed socket/sk.

KASAN reports the race as:

  BUG: KASAN: slab-use-after-free in rds_tcp_skbuf_handler+0x2aa/0x2e0
  rds_tcp_skbuf_handler              net/rds/tcp.c:721
  proc_sys_call_handler              fs/proc/proc_sysctl.c
  vfs_write                          fs/read_write.c
  __x64_sys_pwrite64                 fs/read_write.c

Fix this by unregistering the RDS TCP sysctl table before calling
rds_tcp_kill_sock().  unregister_net_sysctl_table() prevents new sysctl
handlers from starting and waits for in-flight handlers to finish, so
the listen socket can then be released safely. The fix was tested
against the linked reproducer.

Fixes: 7f5611cbc487 ("rds: sysctl: rds_tcp_{rcv,snd}buf: avoid using current->nsproxy")
Reported-by: AutonomousCodeSecurity@microsoft.com
Link: https://lore.kernel.org/all/20260719203718.9680-1-blbllhy@gmail.com
Reviewed-by: Allison Henderson <achender@kernel.org>
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Link: https://patch.msgid.link/20260719210357.10179-1-blbllhy@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>