This patch removes radix tree after finishing tracing IOs.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

This patch adds spin_lock to cover radix tree operations in IO tracer.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

This patch adds two key functions to trace process ids and IOs.
The basic idea is to
1. remain process ids, pids, in page->private.
2. show pids in IO traces.

So, later we can retrieve process information according to IO traces.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

This patch adds:
 o initial trace.c and trace.h with skeleton functions
 o Kconfig and Makefile to activate this feature
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>