Fix divide-by-zero in mmp_delay_update()

vdev_count_leaves() in the denominator may return 0, caught by Coverity.
Introduced by

• 533ea04 Update mmp_delay on sync or skipped, failed write

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #7391