tracing: Add printf attribute to log function

Attributing the function allows the compiler to more thoroughly
check the use of the function with -Wformat and similar flags.

Link: https://lkml.kernel.org/r/20201221162715.3757291-1-trix@redhat.com



Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>