fs: remove inode::i_verity_info

Now that all fsverity-capable filesystems store the pointer to
fsverity_info in the filesystem-specific part of the inode structure,
inode::i_verity_info is no longer needed. Update fsverity_info_addr()
to no longer support the fallback to inode::i_verity_info. Finally,
remove inode::i_verity_info itself, and move the forward declaration of
struct fsverity_info from fs.h (which no longer needs it) to fsverity.h.

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

Co-developed-by: Christian Brauner <[e-mail]>
Signed-off-by: Eric Biggers <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

include/linux/fs.h

···
 struct workqueue_struct;
 struct iov_iter;
 struct fscrypt_operations;
-struct fsverity_info;
 struct fsverity_operations;
 struct fsnotify_mark_connector;
 struct fsnotify_sb_info;
···
 	__u32			i_fsnotify_mask; /* all events this inode cares about */
 	/* 32-bit hole reserved for expanding i_fsnotify_mask */
 	struct fsnotify_mark_connector __rcu	*i_fsnotify_marks;
-#endif
-
-#ifdef CONFIG_FS_VERITY
-	struct fsverity_info	*i_verity_info;
 #endif
 
 	void			*i_private; /* fs or device private pointer */

include/linux/fsverity.h

···
 /* Arbitrary limit to bound the kmalloc() size.  Can be changed. */
 #define FS_VERITY_MAX_DESCRIPTOR_SIZE	16384
 
+struct fsverity_info;
+
 /* Verity operations for filesystems */
 struct fsverity_operations {
 	/**
···
 
 #ifdef CONFIG_FS_VERITY
 
+/*
+ * Returns the address of the verity info pointer within the filesystem-specific
+ * part of the inode.  (To save memory on filesystems that don't support
+ * fsverity, a field in 'struct inode' itself is no longer used.)
+ */
 static inline struct fsverity_info **
 fsverity_info_addr(const struct inode *inode)
 {
-	if (inode->i_sb->s_vop->inode_info_offs == 0)
-		return (struct fsverity_info **)&inode->i_verity_info;
+	VFS_WARN_ON_ONCE(inode->i_sb->s_vop->inode_info_offs == 0);
 	return (void *)inode + inode->i_sb->s_vop->inode_info_offs;
 }