Support for atomic IOs

Hi everyone,

I'm still testing this (especially to look for unrelated regressions in
MD and DM), but wanted to send it out for comments.

These two patches implement support for atomic IOs down to the hardware.
The basic idea is that we give a list of bios to the storage and even
if they are discontiguous the hardware promises to complete them as a
single atomic unit.

The first user for this is O_DIRECT, especially for mysql based
databases.  O_ATOMIC | O_DIRECT allows mysql and friends to disable
double buffering.  This cuts their write IO in half, making them
roughly 2x more flash friendly.

The patches are on top of Jens' current blk-mq/core tree.  Jens
also has chaining patches from Kent, but he is chaining bios likely to
be sent to separate devices, while I'm chaining bios that must
(currently) be sent to the same device as a group.

As far as I can tell, there isn't much overlap in our targets or
methods, but I'd like to eventually merge the ideas together.

-chris

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