tracing: Parse filter from boot event string

Make it possible to specify a trace event filter from the
kernel command line.

Example:

	trace_event=syscalls:sys_enter_clock_nanosleep:filter:which_clock==0

Link: https://patch.msgid.link/20260908-tracing-cli-event-filter-v2-7-05396a3fb663@linutronix.de
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>