btrfs: move verity info pointer to fs-specific part of inode

Move the fsverity_info pointer into the filesystem-specific part of the
inode by adding the field btrfs_inode::i_verity_info and configuring
fsverity_operations::inode_info_offs accordingly.

This is a prerequisite for a later commit that removes
inode::i_verity_info, saving memory and improving cache efficiency on
filesystems that don't support fsverity.

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]>

fs/btrfs/btrfs_inode.h

···
 	struct list_head delayed_iput;
 
 	struct rw_semaphore i_mmap_lock;
+
+#ifdef CONFIG_FS_VERITY
+	struct fsverity_info *i_verity_info;
+#endif
+
 	struct inode vfs_inode;
 };
 

fs/btrfs/inode.c

···
 	struct btrfs_inode *ei = foo;
 
 	inode_init_once(&ei->vfs_inode);
+#ifdef CONFIG_FS_VERITY
+	ei->i_verity_info = NULL;
+#endif
 }
 
 void __cold btrfs_destroy_cachep(void)

fs/btrfs/verity.c

···
 }
 
 const struct fsverity_operations btrfs_verityops = {
+	.inode_info_offs         = (int)offsetof(struct btrfs_inode, i_verity_info) -
+				   (int)offsetof(struct btrfs_inode, vfs_inode),
 	.begin_enable_verity     = btrfs_begin_enable_verity,
 	.end_enable_verity       = btrfs_end_enable_verity,
 	.get_verity_descriptor   = btrfs_get_verity_descriptor,