fuse: Add initial support for fs-verity

This adds support for the FS_IOC_ENABLE_VERITY and FS_IOC_MEASURE_VERITY
ioctls. The FS_IOC_READ_VERITY_METADATA is missing but from the
documentation, "This is a fairly specialized use case, and most fs-verity
users won’t need this ioctl."

Signed-off-by: Richard Fung <richardfung@google.com>
Acked-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>