fscrypt: export fscrypt_d_revalidate()

Dentries that represent no-key names must have a dentry_operations that
includes fscrypt_d_revalidate().  Currently, this is handled by
fscrypt_prepare_lookup() installing fscrypt_d_ops.

However, ceph support for encryption
(https://lore.kernel.org/r/20200914191707.380444-1-jlayton@kernel.org)
can't use fscrypt_d_ops, since ceph already has its own
dentry_operations.

Similarly, ext4 and f2fs support for directories that are both encrypted
and casefolded
(https://lore.kernel.org/r/20200923010151.69506-1-drosen@google.com)
can't use fscrypt_d_ops either, since casefolding requires some dentry
operations too.

To satisfy both users, we need to move the responsibility of installing
the dentry_operations to filesystems.

In preparation for this, export fscrypt_d_revalidate() and give it a
!CONFIG_FS_ENCRYPTION stub.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20200924054721.187797-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

include/linux/fscrypt.h

@@ -197,6 +197,7 @@ int fscrypt_fname_disk_to_usr(const struct inode *inode,
 bool fscrypt_match_name(const struct fscrypt_name *fname,
 			const u8 *de_name, u32 de_name_len);
 u64 fscrypt_fname_siphash(const struct inode *dir, const struct qstr *name);
+int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags);
 
 /* bio.c */
 void fscrypt_decrypt_bio(struct bio *bio);
@@ -454,6 +455,12 @@ static inline u64 fscrypt_fname_siphash(const struct inode *dir,
 	return 0;
 }
 
+static inline int fscrypt_d_revalidate(struct dentry *dentry,
+				       unsigned int flags)
+{
+	return 1;
+}
+
 /* bio.c */
 static inline void fscrypt_decrypt_bio(struct bio *bio)
 {