dax: Avoid ABI change in 4.13.5

dax: Avoid ABI change in 4.13.5

Commit c3ca015fab6d ("dax: remove the pmem_dax_ops->flush
abstraction") removed dax_operations::flush and
target_type::dax_flush, resulting in an ABI change.  Add these
operations back but don't restore any of the calls to them.  To keep
existing callers working during an incomplete kernel upgrade, change
all the implementations to directly do arch_wb_cache_pmem(), just as
dax_flush() does in the new kernel.

Don't change dax_flush() back; it shouldn't have any out-of-tree
callers.

Gbp-Pq: Topic debian
Gbp-Pq: Name dax-avoid-abi-change-in-4.13.5.patch