printk: console_flush_all: use srcu console list iterator

Guarantee safe iteration of the console list by using SRCU.

Note that in the case of a handover, the SRCU read lock is also
released. This is documented in the function description and as
comments in the code. It is a bit tricky, but this preserves the
lockdep lock ordering for the context handing over the
console_lock:

  console_lock()
  | mutex_acquire(&console_lock_dep_map)       <-- console lock
  |
  console_unlock()
  | console_flush_all()
  | | srcu_read_lock(&console_srcu)            <-- srcu lock
  | | console_emit_next_record()
  | | | console_lock_spinning_disable_and_check()
  | | | | srcu_read_unlock(&console_srcu)      <-- srcu unlock
  | | | | mutex_release(&console_lock_dep_map) <-- console unlock

Signed-off-by: John Ogness <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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