Illumos #1043: Recursive zfs snapshot destroy fails

Illumos #1043: Recursive zfs snapshot destroy fails

Prior to revision 11314 if a user was recursively destroying
snapshots of a dataset the target dataset was not required to
exist.  The zfs_secpolicy_destroy_snaps() function introduced
the security check on the target dataset, so since then if the
target dataset does not exist, the recursive destroy is not
performed.  Before 11314, only a delete permission check on
the snapshot's master dataset was performed.

Steps to reproduce:
zfs create pool/a
zfs snapshot pool/a@s1
zfs destroy -r pool@s1

Therefore I suggest to fallback to the old security check, if
the target snapshot does not exist and continue with the destroy.

References to Illumos issue and patch:
- https://www.illumos.org/issues/1043
- https://www.illumos.org/attachments/217/recursive_dataset_destroy.patch

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #340