rds: synchronize info callbacks with module unload

rds_info_getsockopt() reads a callback from rds_info_funcs and invokes it
without protecting the callback's lifetime.  Transport modules register
functions stored in this array.  For example, rds_tcp.ko registers
rds_tcp_tc_info() for RDS_INFO_TCP_SOCKETS.

This permits the following interleaving:

  CPU0                             CPU1
  rds_info_getsockopt()
    func = rds_tcp_tc_info
                                   rmmod rds_tcp
                                     rds_tcp_exit()
                                       rds_info_deregister_func()
                                         rds_info_funcs[offset] = NULL
                                     free rds_tcp module text
    func()

The reader can therefore branch to an address in unloaded module text.

Protect callback invocation with SRCU.  Enter the SRCU read-side critical
section before loading the callback and leave it only after the callback
returns.  Clear the callback with WRITE_ONCE() and call
synchronize_srcu() before deregistration returns, preventing module unload
from freeing its text while an old reader is still executing it.  SRCU is
required because callbacks such as RDS_INFO_COUNTERS can sleep.

Keep the callback array unannotated and use READ_ONCE() and WRITE_ONCE()
for concurrent slot access so sparse does not have to apply __rcu through
the function-pointer typedef.  Replace the two callback-slot BUG_ON()
checks with WARN_ON_ONCE() and return without changing the slot on
mismatch.

Link: https://lore.kernel.org/netdev/20260720184955.3008978-1-nicoyip.dev@gmail.com/
Suggested-by: Allison Henderson <achender@kernel.org>
Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Allison Henderson <achender@kernel.org>
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Link: https://patch.msgid.link/20260801054234.3535077-1-nicoyip.dev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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