Jan Kiszka authored Jul 31, 2017 and Philippe Gerum committed Jul 03, 2018

This allows to inject a user-defined string into the system's ftrace
without leaving RT mode (because of using a standard write on
/sys/kernel/debug/tracing/trace_marker).

As the signature of this function is different from the existing trace
syscall, create as dedicated one.

For simplicity reasons, the maximum string length that can be passed
down to the kernel is limited to 255 characters (+1 for termination).

We call directly into the internal __trace_puts to avoid both the
unneeded strlen call of the trace_puts wrapper and the false warning
that kernel code uses trace_printk.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>