dax: add a facility to lookup a dax device by 'host' device name

For the current block_device based filesystem-dax path, we need a way
for it to lookup the dax_device associated with a block_device. Add a
'host' property of a dax_device that can be used for this purpose. It is
a free form string, but for a dax_device associated with a block device
it is the bdev name.

This is a stop-gap until filesystems are able to mount on a dax-inode
directly.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

include/linux/dax.h

@@ -10,6 +10,7 @@ struct iomap_ops;
 
 int dax_read_lock(void);
 void dax_read_unlock(int id);
+struct dax_device *dax_get_by_host(const char *host);
 
 /*
  * We use lowest available bit in exceptional entry for locking, one bit for