tracing: Add skip argument to trace_dump_stack()

Altough the trace_dump_stack() already skips three functions in
the call to stack trace, which gets the stack trace to start
at the caller of the function, the caller may want to skip some
more too (as it may have helper functions).

Add a skip argument to the trace_dump_stack() that lets the caller
skip back tracing functions that it doesn't care about.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>