Impact: rename of debugfs file trace_entries to buffer_size_kb

The original ftrace had fixed size entries, and the number of entries
was shown and modified via the file called trace_entries. By converting
to the unified trace buffer, we now allow for variable size entries
which makes the meaning of trace_entries pointless.

Since trace_size might be confused to the size of the trace, this patch
names it "buffer_size_kb" (thanks to Arjan van de Ven for this idea).

[ mingo@elte.hu: changed from buffer_size to buffer_size_kb ]

( Note, the units are still bytes - the next patch changes that,
  to keep the wide rename patch separate from the unit-change patch. )
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>