Fix typo/etc in module/zfs/zfs_ctldir.c

Fix typo/etc in module/zfs/zfs_ctldir.c

Drop duplicated phrases in comments.

Also drop an obsolete comment "Perform a mount of the associated...",
as all it does now is get objid from DMU and lookup incore inode.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8707