tracing/user_events: Don't destroy fields when event removal fails

destroy_user_event() destroys the event's fields before attempting to
remove the trace event call. If user_event_set_call_visible() fails,
e.g. because the event is still enabled and trace_remove_event_call()
returns -EBUSY, the event is left registered with an irreversibly
destroyed field list. Any subsequent interaction with the event then
operates on an empty field list while it is still fully visible in
tracefs.

Move the field destruction after the call removal, and splice the
field list back onto the event when the removal fails so the event
remains in a consistent state.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260904115223.2976446-1-bsdhenrymartin@gmail.com
Fixes: 7f5a08c79df35 ("user_events: Add minimal support for trace_event into ftrace")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
Reviewed-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>