dm: add asymmetric stripe device driver

Device-Mapper's "asm-striped" target is used to create a striped (i.e. RAID-0) device across one or more underlying devices. Data is written in "chunks", with consecutive chunks rotating among the underlying devices. This can potentially provide improved I/O throughput by utilizing several physical devices in parallel. However, in order to gain maximum I/O performance between slow and fast device, there is a ratio to set up the chunk size among these device.

Parameters: <num devs> <chunk size> <ratio> [<dev path> <offset>]+ <num devs>: Number of underlying devices.
<chunk size>: Size of each chunk of data. Must be at least as large as the system's PAGE_SIZE.
<ratio>: The proportion of per io size, it is the times as much as 1 chunk size
<dev path>: Full pathname to the underlying block-device, or a "major:minor" device-number.
<offset>: Starting sector within the device.

 Documentation/device-mapper/asymmetric-striped.txt |  85 ++++
 drivers/md/Kconfig                                 |  11 +
 drivers/md/Makefile                                |   1 +
 drivers/md/dm-asymmetric-stripe.c                  | 556 +++++++++++++++++++++
 drivers/md/dm.c                                    |   5 +
 include/linux/device-mapper.h                      |  15 +
 6 files changed, 678 insertions(+)