Skip snapshot in zfs_iter_mounted()

The intention of the zfs_iter_mounted() is to traverse the dataset
and its descendants, not the snapshots. The current code can cause
a mounted snapshot to be included and thus zfs_open() on the snapshot
with ZFS_TYPE_FILESYSTEM would print confusing message such as "cannot
open 'rpool/fs@snap': snapshot delimiter '@' is not expected here".

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Youzhong Yang <yyang@mathworks.com>
Closes #12447
Closes #12448