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