generic: test revalidation of encrypted dentries

generic: test revalidation of encrypted dentries

Add a test which verifies that dentries in an encrypted directory
are invalidated when an encryption key is added --- which should
cause the plaintext filenames to be visible and accessible,
replacing the encoded ciphertext filenames and any negative dentries
for the plaintext names.  This primarily tests for a bug which was
fixed in the v4.5 kernel, plus a v4.6 fix for incorrect RCU usage in
the earlier fix.

Cc: linux-fscrypt@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>