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:

1. 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:

1. 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.