fscrypt: don't allow v1 policies with casefolding

Casefolded encrypted directories will use a new dirhash method that
requires a secret key.  If the directory uses a v2 encryption policy,
it's easy to derive this key from the master key using HKDF.  However,
v1 encryption policies don't provide a way to derive additional keys.

Therefore, don't allow casefolding on directories that use a v1 policy.
Specifically, make it so that trying to enable casefolding on a
directory that has a v1 policy fails, trying to set a v1 policy on a
casefolded directory fails, and trying to open a casefolded directory
that has a v1 policy (if one somehow exists on-disk) fails.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
[EB: improved commit message, updated fscrypt.rst, and other cleanups]
Link: https://lore.kernel.org/r/20200120223201.241390-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/filesystems/fscrypt.rst

@@ -513,7 +513,9 @@ FS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors:
 - ``EEXIST``: the file is already encrypted with an encryption policy
   different from the one specified
 - ``EINVAL``: an invalid encryption policy was specified (invalid
-  version, mode(s), or flags; or reserved bits were set)
+  version, mode(s), or flags; or reserved bits were set); or a v1
+  encryption policy was specified but the directory has the casefold
+  flag enabled (casefolding is incompatible with v1 policies).
 - ``ENOKEY``: a v2 encryption policy was specified, but the key with
   the specified ``master_key_identifier`` has not been added, nor does
   the process have the CAP_FOWNER capability in the initial user