OpenZFS 7900 - zdb shouldn't print the path of a znode at verbosity < 5

OpenZFS 7900 - zdb shouldn't print the path of a znode at verbosity < 5

Authored by: Alan Somers <asomers@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
There are two reasons:
1) Finding a znode's path is slower than printing any other znode
   information at verbosity < 5.
2) On a corrupted pool like the one mentioned below, zdb will crash when it
   tries to determine the znode's path. But with this patch, zdb can still
   extract useful information from such pools.

OpenZFS-issue: https://www.illumos.org/issues/7900
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/2b0dee1
Closes #6016