tracing: Add dynamic event command creation interface

tracing: Add dynamic event command creation interface

Add an interface used to build up dynamic event creation commands,
such as synthetic and kprobe events.  Interfaces specific to those
particular types of events and others can be built on top of this
interface.

Command creation is started by first using the dynevent_cmd_init()
function to initialize the dynevent_cmd object.  Following that, args
are appended and optionally checked by the dynevent_arg_add() and
dynevent_arg_pair_add() functions, which use objects representing
arguments and pairs of arguments, initialized respectively by
dynevent_arg_init() and dynevent_arg_pair_init().  Finally, once all
args have been successfully added, the command is finalized and
actually created using dynevent_create().

The code here for actually printing into the dyn_event->cmd buffer
using snprintf() etc was adapted from v4 of Masami's 'tracing/boot:
Add synthetic event support' patch.

Link: http://lkml.kernel.org/r/1f65fa44390b6f238f6036777c3784ced1dcc6a0.1580323897.git.zanussi@kernel.org
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>