ZTS: Fix import_devices_missing.ksh

Related to commit 90b77a0.  Retry the `zpool export` if the pool
is "busy" indicating there is a process accessing the mount point.
This can happen after an import, allowing it to be retried will
avoid spurious test failures.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#13169

tests/zfs-tests/tests/functional/cli_root/zpool_import/import_devices_missing.ksh

@@ -68,7 +68,7 @@ function test_devices_missing
 
 	log_must generate_data $TESTPOOL1 $MD5FILE2 "second"
 
-	log_must zpool export $TESTPOOL1
+	log_must_busy zpool export $TESTPOOL1
 
 	log_must mv $missingvdevs $BACKUP_DEVICE_DIR
 
@@ -85,7 +85,7 @@ function test_devices_missing
 	    "get suspended."
 	verify_data_md5sums $MD5FILE >/dev/null 2>&1
 
-	log_must zpool export $TESTPOOL1
+	log_must_busy zpool export $TESTPOOL1
 
 	typeset newpaths=$(echo "$missingvdevs" | \
 		sed "s:$DEVICE_DIR:$BACKUP_DEVICE_DIR:g")