dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c

dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c

This function will be used by both device.c and fsdev.c, but both are
loadable modules. Moving to bus.c puts it in core and makes it available
to both.

No code changes - just relocated.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: John Groves <john@groves.net>
Link: https://patch.msgid.link/0100019d311c90eb-a582ff97-93ba-49f3-8140-6c5c4bf8bc62-000000@email.amazonses.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>