Fix zdb calling behavior in ztest
The current zdb calling behaviour is really fragile, and is guaranteed to
segfault if ztest is not installed in either /sbin or /usr/sbin. With this
patch, the ztest will try to call zdb in the following order.
- Use environmental variable ZDB_PATH if provided.
- If ztest resides in build tree, guess the in tree zdb path.
- Just pass zdb to popen and let it search it in PATH.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3126