generic: test for buggy fscrypt context consistency check

generic: test for buggy fscrypt context consistency check

Add a regression test for a bug where ->lookup() in an encrypted
directory would incorrectly return EPERM, depending on which inodes
happened to have their keys still cached in memory following removal of
the keyring key.  This bug was fixed in v4.12-rc1, v4.9.29, and v4.4.70.

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>