Fix 'zfs receive -F' message when destination has snapshots

Fix 'zfs receive -F' message when destination has snapshots

When receiving a send stream with forced rollback on a dataset with
snapshots zfs suggests said snapshots must be removed to successfully
receive the stream; however the message is misleading because it
prints the dataset name instead of one of its snapshots.

   $ sudo zfs snap pp/recvfs@snap-orig
   $ sudo zfs recv -F pp/recvfs < sendstream
   cannot receive new filesystem stream: destination has snapshots (eg. pp/recvfs)
   must destroy them to overwrite it

This change simply restores the snapshot name in the error message.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8167