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

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 <brauner@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/20250810075706.172910-12-ebiggers@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>