ZTS: Fix rollback_003_pos.ksh

Under Linux when rolling back a mounted filesystem negative dentries
may not be dropped from the cache. This can result in an ENOENT
being incorrectly returned on first access. Issuing a df before
the unmount results in the negative dentries being invalidated and
side steps the issue.

This is solely a workaround for the test case on Linux and not
correct behavior. The core issue of invalidating negative dentries
needs to be handled with a kernel side change. This is being
tracked as issue #6143.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12898
Issue #6143