Get disk device information.
Currently this only includes the
disk device address that corresponds
-to the current file offset.
+to the current file offset. Note that if the
+file offset is not backed by physical blocks
+we can return -1 as the offset. This is subject
+to change.
.It Dv F_LOG2PHYS_EXT
Variant of F_LOG2PHYS that uses the passed in
file offset and length.