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

(cherry picked from commit d2d5a0ea03b0d461a4d376c7b9de8cd5c147effa)
(cherry picked from commit 6ddd9eff78a15081dfe3b405b3d0a958ff27acf5)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
HDFS-7181. Remove incorrect precondition check on key length in FileEncryptionInfo. (wang)

(cherry picked from commit 6ac10516e7fa28384b6d3c2670f6621e2666ffdb)
(cherry picked from commit 6dbba19469d698891e9c6a8513cbe58992a736b1)