fsverity: add support for info in fs-specific part of inode

fsverity: add support for info in fs-specific part of inode

Add an inode_info_offs field to struct fsverity_operations, and update
fs/verity/ to support it.  When set to a nonzero value, it specifies the
offset to the fsverity_info pointer within the filesystem-specific part
of the inode structure, to be used instead of inode::i_verity_info.

Since this makes inode::i_verity_info no longer necessarily used, update
comments that mentioned it.

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/20250810075706.172910-9-ebiggers@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>