fs-verity: add inode and superblock fields

Analogous to fs/crypto/, add fields to the VFS inode and superblock for
use by the fs/verity/ support layer:

- ->s_vop: points to the fsverity_operations if the filesystem supports
  fs-verity, otherwise is NULL.

- ->i_verity_info: points to cached fs-verity information for the inode
  after someone opens it, otherwise is NULL.

- S_VERITY: bit in ->i_flags that identifies verity inodes, even when
  they haven't been opened yet and thus still have NULL ->i_verity_info.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>