Fix minor issues in namespace delegation support

get_user_ns() is only done once for each namespace, so put_user_ns()
should be done once too.

Fix two typos in user_namespace/user_namespace_002.ksh and
user_namespace/user_namespace_003.ksh.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Youzhong Yang <yyang@mathworks.com>
Closes #13918