ZTS: Fix zfs_share_* test case failures

Prevent false positives when running the zfs_share_* test
cases due to leftover stale /var/lib/nfs/etab entries. When
starting the test group re-synchronize the /var/lib/nfs/etab
file with /etc/exports. At this point in the testing there
will be no additional zfs share entries to add.

Reviewed by: George Melikov <mail@gmelikov.ru>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7226