fs/crypto: do not clear encryption key before inode is updated

Although an all zero key is not an invalid key but it is indeed a
weak key. Clearing crypt key memory before updating the file inode with
crypto information will lead to a weak file encryption key to be
used in encryption operation.

Change-Id: Ide6340ea58f7ff916d3161716f0e9b4adf9b9c75
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>