Disable user space reference tracking

Disable user space reference tracking

The memory and cpu cost of reference count tracking with the current
implementation is significant.  For this reason it has always been
disabled by default for the kmods.  Apply this same default to user
space so ztest doesn't always incur this performance penalty.

Our intention is to re-enable this by default for ztest once the code
has been optimized.  Since we expect to at some point provide a FUSE
implementation we wouldn't want this enabled by default for libzpool.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10189