user_events: Validate user payloads for size and null termination

Add validation to ensure data is at or greater than the min size for the
fields of the event. If a dynamic array is used and is a type of char,
ensure null termination of the array exists.

Link: https://lkml.kernel.org/r/20220118204326.2169-7-beaub@linux.microsoft.com



Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>