ring-buffer: Show persistent buffer dropped events in trace file

When the persistent ring buffer is validated on boot up, if a subbuffer is
deemed invalid, it resets the buffer and continues. Currently, these lost
events are not shown in the trace file output.

Have the trace iterator look for subbuffers that have the RB_MISSED_EVENTS
set and set the iter->missed_events flag when it is detected. This will
then have the trace file shows "LOST EVENTS" when it reads across a
subbuffer that was corrupted and invalidated.

For example:

 <...>-1016    [005] ...1.  6230.660403: preempt_disable: caller=__mod_memcg_state+0x1c8/0x200 parent=__mod_memcg_state+0x1c8/0x200
CPU:5 [LOST EVENTS]
 <...>-1016    [005] .....  6230.660673: kmem_cache_alloc: call_site=__anon_vma_prepare+0x1ad/0x1e0 ptr=000000006e40294c name=anon_vma bytes_req=200 bytes_alloc=208 gfp_flags=GFP_KERNEL node=-1 accounted=true

Link: https://patch.msgid.link/20260522171052.006276604@kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

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