fs-verity: implement FS_IOC_MEASURE_VERITY ioctl

Add a function for filesystems to call to implement the
FS_IOC_MEASURE_VERITY ioctl.  This ioctl retrieves the file measurement
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 authentication or logging of file measurements in userspace.

See the "FS_IOC_MEASURE_VERITY" section of
Documentation/filesystems/fsverity.rst for the documentation.

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