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

tests/zfs-tests/tests/functional/mmp/mmp.kshlib

@@ -198,6 +198,6 @@ function count_uberblocks # pool duration
 	typeset -i duration=$2
 	typeset hist_path="/proc/spl/kstat/zfs/$pool/multihost"
 
-	log_must sleep $duration
+	sleep $duration
 	echo $(cat "$hist_path" | sed '1,2d' | wc -l)
 }