Fix dataset name comparison in zfs_compare().

Fix dataset name comparison in zfs_compare().

The code never returned match comparing two datasets (not snapshots).
As result, uu_avl_find(), called from zfs_callback(), never succeeded,
allowing to add same dataset into the list multiple times, for example:

	# zfs get name pers pers pers@z pers@z
	NAME    PROPERTY  VALUE   SOURCE
	pers    name      pers    -
	pers    name      pers    -
	pers@z  name      pers@z  -

With the patch:

	# zfs get name pers pers pers@z pers@z
	NAME    PROPERTY  VALUE   SOURCE
	pers    name      pers    -
	pers@z  name      pers@z  -

MFC after:	1 week
Sponsored by:	iXsystems, Inc.