Disable vdev_zaps_004_pos

Occasionally observed failure of vdev_zaps_004_pos due to the test
case not being 100% reliable. In order to prevent false positives
disable this test case until it can be made reliable.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #6935
Closes #6936