fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds

In the Linux kernel, the following vulnerability has been resolved:

fsverity: reject FS_IOC_ENABLE_VERITY on mode 3 fds

Commit 56124d6c87fd ("fsverity: support enabling with tree block size <

PAGE_SIZE") changed FS_IOC_ENABLE_VERITY to use __kernel_read() to read

the file's data, instead of direct pagecache accesses.

An unintended consequence of this is that the

'WARN_ON_ONCE(!(file->f_mode & FMODE_READ))' in __kernel_read() became

reachable by fuzz tests. This happens if FS_IOC_ENABLE_VERITY is called

on a fd opened with access mode 3, which means "ioctl access only".

Arguably, FS_IOC_ENABLE_VERITY should work on ioctl-only fds. But

ioctl-only fds are a weird Linux extension that is rarely used and that

few people even know about. (The documentation for FS_IOC_ENABLE_VERITY

even specifically says it requires O_RDONLY.) It's probably not

worthwhile to make the ioctl internally open a new fd just to handle

this case. Thus, just reject the ioctl on such fds for now.