Fix traverse_impl() kmem leak

Fix traverse_impl() kmem leak

The error path must free the memory allocated by this function or
it will be leaked.  In practice, this would leak only a few bytes
of memory under rare circumstances and thus is unlikely to have
caused any real problems.  This issue was caught by the kmemleak.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7791