eventfs: Hold eventfs_mutex and SRCU when remount walks events

Commit 340f0c7067a9 ("eventfs: Update all the eventfs_inodes from the
events descriptor") had eventfs_set_attrs() recurse through ei->children
on remount.  The walk only holds the rcu_read_lock() taken by
tracefs_apply_options() over tracefs_inodes, which is wrong:

  - list_for_each_entry over ei->children races with the list_del_rcu()
    in eventfs_remove_rec() -- LIST_POISON1 deref, same shape as
    d2603279c7d6.
  - eventfs_inodes are freed via call_srcu(&eventfs_srcu, ...).
    rcu_read_lock() does not extend an SRCU grace period, so ti->private
    can be reclaimed under the walk.
  - The writes to ei->attr race with eventfs_set_attr(), which holds
    eventfs_mutex.

Reproducer:

  while :; do mount -o remount,uid=$((RANDOM%1000)) /sys/kernel/tracing; done &
  while :; do
      echo "p:kp submit_bio" > /sys/kernel/tracing/kprobe_events
      echo > /sys/kernel/tracing/kprobe_events
  done

Wrap the events portion of tracefs_apply_options() in
eventfs_remount_lock()/_unlock() that take eventfs_mutex and
srcu_read_lock(&eventfs_srcu).  eventfs_set_attrs() doesn't sleep so the
nested rcu_read_lock() is fine; lockdep_assert_held() pins the contract.

Comment in tracefs_drop_inode() said "RCU cycle" -- it is SRCU.

Fixes: 340f0c7067a9 ("eventfs: Update all the eventfs_inodes from the events descriptor")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260418191737.10289-1-devnexen@gmail.com
Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>