fs: remove inode::i_crypt_info

Now that all fscrypt-capable filesystems store the pointer to
fscrypt_inode_info in the filesystem-specific part of the inode
structure, inode::i_crypt_info is no longer needed.  Update
fscrypt_inode_info_addr() to no longer support the fallback to
inode::i_crypt_info.  Finally, remove inode::i_crypt_info itself along
with the now-unnecessary forward declaration of fscrypt_inode_info.

The end result of the migration to the filesystem-specific pointer is
memory savings on CONFIG_FS_ENCRYPTION=y kernels for all filesystems
that don't support fscrypt.  Specifically, their in-memory inodes are
now smaller by the size of a pointer: either 4 or 8 bytes.

Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/20250810075706.172910-8-ebiggers@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Eric Biggers

include/linux/fs.h

@@ -72,7 +72,6 @@ struct swap_info_struct;
 struct seq_file;
 struct workqueue_struct;
 struct iov_iter;
-struct fscrypt_inode_info;
 struct fscrypt_operations;
 struct fsverity_info;
 struct fsverity_operations;
@@ -780,10 +779,6 @@ struct inode {
 	struct fsnotify_mark_connector __rcu	*i_fsnotify_marks;
 #endif
 
-#ifdef CONFIG_FS_ENCRYPTION
-	struct fscrypt_inode_info	*i_crypt_info;
-#endif
-
 #ifdef CONFIG_FS_VERITY
 	struct fsverity_info	*i_verity_info;
 #endif

include/linux/fscrypt.h

@@ -201,11 +201,15 @@ struct fscrypt_operations {
 int fscrypt_d_revalidate(struct inode *dir, const struct qstr *name,
 			 struct dentry *dentry, unsigned int flags);
 
+/*
+ * Returns the address of the fscrypt info pointer within the
+ * filesystem-specific part of the inode.  (To save memory on filesystems that
+ * don't support fscrypt, a field in 'struct inode' itself is no longer used.)
+ */
 static inline struct fscrypt_inode_info **
 fscrypt_inode_info_addr(const struct inode *inode)
 {
-	if (inode->i_sb->s_cop->inode_info_offs == 0)
-		return (struct fscrypt_inode_info **)&inode->i_crypt_info;
+	VFS_WARN_ON_ONCE(inode->i_sb->s_cop->inode_info_offs == 0);
 	return (void *)inode + inode->i_sb->s_cop->inode_info_offs;
 }