HDFS-7179. DFSClient should instantiate a KeyProvider, not a KeyProviderCryptoExtension. (wang)

(cherry picked from commit d2d5a0ea03b0d461a4d376c7b9de8cd5c147effa)
HDFS-7181. Remove incorrect precondition check on key length in FileEncryptionInfo. (wang)

(cherry picked from commit 6ac10516e7fa28384b6d3c2670f6621e2666ffdb)