fs, fscrypt: add an S_ENCRYPTED inode flag

Introduce a flag S_ENCRYPTED which can be set in ->i_flags to indicate
that the inode is encrypted using the fscrypt (fs/crypto/) mechanism.

Checking this flag will give the same information that
inode->i_sb->s_cop->is_encrypted(inode) currently does, but will be more
efficient.  This will be useful for adding higher-level helper functions
for filesystems to use.  For example we'll be able to replace this:

	if (ext4_encrypted_inode(inode)) {
		ret = fscrypt_get_encryption_info(inode);
		if (ret)
			return ret;
		if (!fscrypt_has_encryption_key(inode))
			return -ENOKEY;
	}

with this:

	ret = fscrypt_require_key(inode);
	if (ret)
		return ret;

... since we'll be able to retain the fast path for unencrypted files as
a single flag check, using an inline function.  This wasn't possible
before because we'd have had to frequently call through the
->i_sb->s_cop->is_encrypted function pointer, even when the encryption
support was disabled or not being used.

Note: we don't define S_ENCRYPTED to 0 if CONFIG_FS_ENCRYPTION is
disabled because we want to continue to return an error if an encrypted
file is accessed without encryption support, rather than pretending that
it is unencrypted.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ubifs/xattr.c

@@ -170,6 +170,7 @@ static int create_xattr(struct ubifs_info *c, struct inode *host,
 	err = ubifs_jnl_update(c, host, nm, inode, 0, 1);
 	if (err)
 		goto out_cancel;
+	ubifs_set_inode_flags(host);
 	mutex_unlock(&host_ui->ui_mutex);
 
 	ubifs_release_budget(c, &req);