tracing: Add interface to allow multiple trace buffers

Add the interface ("instances" directory) to add multiple buffers
to ftrace. To create a new instance, simply do a mkdir in the
instances directory:

This will create a directory with the following:

 # cd instances
 # mkdir foo
 # ls foo
buffer_size_kb        free_buffer  trace_clock    trace_pipe
buffer_total_size_kb  set_event    trace_marker   tracing_enabled
events/               trace        trace_options  tracing_on

Currently only events are able to be set, and there isn't a way
to delete a buffer when one is created (yet).

Note, the i_mutex lock is dropped from the parent "instances"
directory during the mkdir operation. As the "instances" directory
can not be renamed or deleted (created on boot), I do not see
any harm in dropping the lock. The creation of the sub directories
is protected by trace_types_lock mutex, which only lets one
instance get into the code path at a time. If two tasks try to
create or delete directories of the same name, only one will occur
and the other will fail with -EEXIST.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>