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

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>