tracing: Have trace_buffer_unlock_commit() call the _regs version with NULL

There's no real difference between trace_buffer_unlock_commit() and
trace_buffer_unlock_commit_regs() except that the former passes NULL to
ftrace_stack_trace() instead of regs. Have the former be a static inline of
the latter which passes NULL for regs.

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