Correct count_uberblocks in mmp.kshlib

A log_must call was causing count_uberblocks to return more
than just the uberblock count. Remove the log_must since it
was only logging a sleep.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #7191