Refactor parent dataset handling in libzfs zfs_rename()

For recursive renaming, simplify the code by moving zhrp and
parentname to inner scope. zhrp is only used to test existence
of a parent dataset for recursive dataset dir scan since ba6a24026c.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8815