dax: introduce dax_operations

Track a set of dax_operations per dax_device that can be set at
alloc_dax() time. These operations will be used to stop the abuse of
block_device_operations for communicating dax capabilities to
filesystems. It will also be used to replace the "pmem api" and move
pmem-specific cache maintenance, and other dax-driver-specific
filesystem-dax operations, to dax device methods. In particular this
allows us to stop abusing __copy_user_nocache(), via memcpy_to_pmem(),
with a driver specific replacement.

This is a standalone introduction of the operations. Follow on patches
convert each dax-driver and teach fs/dax.c to use ->direct_access() from
dax_operations instead of block_device_operations.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

include/linux/dax.h

@@ -7,6 +7,16 @@
 #include <asm/pgtable.h>
 
 struct iomap_ops;
+struct dax_device;
+struct dax_operations {
+	/*
+	 * direct_access: translate a device-relative
+	 * logical-page-offset into an absolute physical pfn. Return the
+	 * number of pages available for DAX at that pfn.
+	 */
+	long (*direct_access)(struct dax_device *, pgoff_t, long,
+			void **, pfn_t *);
+};
 
 int dax_read_lock(void);
 void dax_read_unlock(int id);