zfs_ioc_snapshot: check user-prop permissions on snapshotted datasets

Previously, the permissions were checked on the pool which was obviously
incorrect.

After this change, zfs_check_userprops() only validates the properties
without any permission checks. The permissions are checked individually
for each snapshotted dataset.

This was also committed to ZoL: zfsonlinux/zfs@e6203d2

Reported by: CyberSecure
MFC after: 1 week
Sponsored by: CyberSecure