Fix variable shadowing in libzfs_mount

We accidentally reused variable name "i" for inner and outer loops.

Reviewed-by: Rich Ercolani <Rincebrain@gmail.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Reno Reckling <e-github@wthack.de>
Closes #14452
Closes #14445