fs: remove inode::i_verity_info

fs: remove inode::i_verity_info

Now that all fsverity-capable filesystems store the pointer to
fsverity_info in the filesystem-specific part of the inode structure,
inode::i_verity_info is no longer needed.  Update fsverity_info_addr()
to no longer support the fallback to inode::i_verity_info.  Finally,
remove inode::i_verity_info itself, and move the forward declaration of
struct fsverity_info from fs.h (which no longer needs it) to fsverity.h.

The end result of the migration to the filesystem-specific pointer is
memory savings on CONFIG_FS_VERITY=y kernels for all filesystems that
don't support fsverity.  Specifically, their in-memory inodes are now
smaller by the size of a pointer: either 4 or 8 bytes.

Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/20250810075706.172910-13-ebiggers@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>