tracing: Make the backup instance non-reusable

tracing: Make the backup instance non-reusable

Since there is no reason to reuse the backup instance, make it readonly
(but erasable).  Note that only backup instances are readonly, because
other trace instances will be empty unless it is writable.  Only backup
instances have copy entries from the original.

With this change, most of the trace control files are removed from the
backup instance, including eventfs enable/filter etc.

 # find /sys/kernel/tracing/instances/backup/events/ | wc -l
 4093
 # find /sys/kernel/tracing/instances/boot_map/events/ | wc -l
 9573

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/177502546939.1311542.1826814401724828930.stgit@mhiramat.tok.corp.google.com
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>