tracing: Add trace_seq_buffer_ptr() helper function

tracing: Add trace_seq_buffer_ptr() helper function

There's several locations in the kernel that open code the calculation
of the next location in the trace_seq buffer. This is usually done with

  p->buffer + p->len

Instead of having this open coded, supply a helper function in the
header to do it for them. This function is called trace_seq_buffer_ptr().

Link: http://lkml.kernel.org/p/20140626220129.452783019@goodmis.org
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>