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  -

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #8723