ext4: move crypt info pointer to fs-specific part of inode

Move the fscrypt_inode_info pointer into the filesystem-specific part of
the inode by adding the field ext4_inode_info::i_crypt_info and
configuring fscrypt_operations::inode_info_offs accordingly.

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

Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/20250810075706.172910-4-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Eric Biggers

fs/ext4/crypto.c

@@ -227,6 +227,8 @@ static bool ext4_has_stable_inodes(struct super_block *sb)
 }
 
 const struct fscrypt_operations ext4_cryptops = {
+	.inode_info_offs	= (int)offsetof(struct ext4_inode_info, i_crypt_info) -
+				  (int)offsetof(struct ext4_inode_info, vfs_inode),
 	.needs_bounce_pages	= 1,
 	.has_32bit_inodes	= 1,
 	.supports_subblock_data_units = 1,

fs/ext4/ext4.h

@@ -1182,6 +1182,10 @@ struct ext4_inode_info {
 	__u32 i_csum_seed;
 
 	kprojid_t i_projid;
+
+#ifdef CONFIG_FS_ENCRYPTION
+	struct fscrypt_inode_info *i_crypt_info;
+#endif
 };
 
 /*

fs/ext4/super.c

@@ -1470,6 +1470,9 @@ static void init_once(void *foo)
 	init_rwsem(&ei->i_data_sem);
 	inode_init_once(&ei->vfs_inode);
 	ext4_fc_init_inode(&ei->vfs_inode);
+#ifdef CONFIG_FS_ENCRYPTION
+	ei->i_crypt_info = NULL;
+#endif
 }
 
 static int __init init_inodecache(void)