Add *_by_dnode() routines for accessing objects given their
dnode_t *, this is more efficient than accessing the object by
(objset_t *, uint64_t object). This change converts some but
not all of the existing consumers. As performance-sensitive
code paths are discovered they should be converted to use
these routines.
Ported from: https://github.com/zfsonlinux/zfs/commit/
0eef1bde31d67091d3deed23fe2394f5a8bf2276