Add a function for filesystems to call to implement the
FS_IOC_MEASURE_VERITY ioctl.  This ioctl retrieves the file measurement
hash that fs-verity calculated for the given file and is enforcing for
reads; i.e., reads that don't match this hash will fail.  This ioctl can
be used for logging or authentication of file hashes in userspace.

This ioctl is documented in Documentation/filesystem/fsverity.rst;
see there for more information.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>