ext4 crypto: revalidate dentry after adding or removing the key

ext4 crypto: revalidate dentry after adding or removing the key

Add a validation check for dentries for encrypted directory to make
sure we're not caching stale data after a key has been added or removed.

Also check to make sure that status of the encryption key is updated
when readdir(2) is executed.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>
Git-commit: b4fd46149cbb118c6c2fa29f24391e79b87862b7
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
Change-Id: I64ea95854de3c633b17bea0b63ab658e1524befb
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>