Return positive error number in zfsctl_shares_lookup.

Otherwise it will cause zpl_shares_lookup() to return a invalid
pointer when an error occurs.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
Closes openzfs#626 openzfs#885 openzfs#947 openzfs#977

module/zfs/zfs_ctldir.c

@@ -952,7 +952,7 @@ zfsctl_shares_lookup(struct inode *dip, char *name, struct inode **ipp,
 
 	if (zsb->z_shares_dir == 0) {
 		ZFS_EXIT(zsb);
-		return (-ENOTSUP);
+		return (ENOTSUP);
 	}
 
 	error = zfs_zget(zsb, zsb->z_shares_dir, &dzp);