ZTS: rsend_007_pos failures

The rsend_007_pos test reliably fails on Linux in the cleanup
function. This is caused by an unmount error when attempting to
recursively destroy the newly received datasets. Invoking df
prior to the zfs destroy interestingly avoids the unmont error.

Why this should matter is unclear and should be investigated.
However, this minor tweak may allow us to remove the ZTS rsend
exceptions. The subsequent rsend_010_pos and rsend_011_pos
failures were a result of this initial failure. The other
"maybe" failures I was unable to reproduce and have not been
recently observed in the master branch.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5665
Closes #6086
Closes #6087
Closes #6446
Closes #12876