Change 3-digit octal escapes to 4-digit ones

Prefixing an octal value with a leading zero is the standard way
to disambiguate it. This change only impacts the zfs diff output
and is therefore very limited in scope.

Signed-off-by: Hajo M<C3><B6>ller <dasjoe@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3417