dax: remove the pmem_dax_ops->flush abstraction

Commit abebfbe2 ("dm: add ->flush() dax operation support") is
buggy. A DM device may be composed of multiple underlying devices and
all of them need to be flushed. That commit just routes the flush
request to the first device and ignores the other devices.

It could be fixed by adding more complex logic to the device mapper. But
there is only one implementation of the method pmem_dax_ops->flush - that
is pmem_dax_flush() - and it calls arch_wb_cache_pmem(). Consequently, we
don't need the pmem_dax_ops->flush abstraction at all, we can call
arch_wb_cache_pmem() directly from dax_flush() because dax_dev->ops->flush
can't ever reach anything different from arch_wb_cache_pmem().

It should be also pointed out that for some uses of persistent memory it
is needed to flush only a very small amount of data (such as 1 cacheline),
and it would be overkill if we go through that device mapper machinery for
a single flushed cache line.

Fix this by removing the pmem_dax_ops->flush abstraction and call
arch_wb_cache_pmem() directly from dax_flush(). Also, remove the device
mapper code that forwards the flushes.

Fixes: abebfbe2 ("dm: add ->flush() dax operation support")
Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>