Fix for mountpoint=legacy

We need to clear mountpoint only after checking it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: ofthesun9 <olivier@ofthesun.net>
Closes #14599
Closes #14604

contrib/initramfs/scripts/zfs

@@ -341,11 +341,12 @@ mount_fs()
 				# isn't the root fs.
 				return 0
 			fi
-			# Last hail-mary: Hope 'rootmnt' is set!
-			mountpoint=""
+			# Don't use mount.zfs -o zfsutils for legacy mountpoint
 			if [ "$mountpoint" = "legacy" ]; then
 				ZFS_CMD="mount.zfs"
 			fi
+			# Last hail-mary: Hope 'rootmnt' is set!
+			mountpoint=""
 		else
 			mountpoint="$mountpoint1"
 		fi