fs, fscrypt: only define ->s_cop when FS_ENCRYPTION is enabled

Now that filesystems only set and use their fscrypt_operations when they
are built with encryption support, we can remove ->s_cop from
'struct super_block' when FS_ENCRYPTION is disabled.  This saves a few
bytes on some kernels and also makes it consistent with ->i_crypt_info.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

include/linux/fs.h

@@ -1364,9 +1364,9 @@ struct super_block {
 	void                    *s_security;
 #endif
 	const struct xattr_handler **s_xattr;
-
+#if IS_ENABLED(CONFIG_FS_ENCRYPTION)
 	const struct fscrypt_operations	*s_cop;
-
+#endif
 	struct hlist_bl_head	s_roots;	/* alternate root dentries for NFS */
 	struct list_head	s_mounts;	/* list of mounts; _not_ for fs use */
 	struct block_device	*s_bdev;