Remove ambiguity on demand vs prefetch stats reported by arc_summary

[shodanshok]

Motivation and Context

arc_summary currently list prefetch stats as "demand prefetch". For example:

Cache hits by data type:
        Demand data:                                   26.3 %      13.6M
        Demand prefetch data:                           0.6 %     323.5k
        Demand metadata:                               73.1 %      37.9M
        Demand prefetch metadata:                     < 0.1 %      15.0k

Cache misses by data type:
        Demand data:                                   65.9 %       1.1M
        Demand prefetch data:                          31.2 %     534.2k
        Demand metadata:                                1.2 %      20.8k
        Demand prefetch metadata:                       1.7 %      28.6k

However, a hit/miss can be due to demand or prefetch, not both at the same time.
This commit clearly marks prefetched data/metadata as such, by removing "Demand" from the line.

Description

The commit is extremely simple, and only affects the arc_summary file in how it prints four rows.

How Has This Been Tested?

On a local machine with the commit applied, arc_summary correctly show the following:

Cache hits by data type:
        Demand data:                                   26.3 %      13.6M
        Prefetch data:                                  0.6 %     323.5k
        Demand metadata:                               73.1 %      37.9M
        Prefetch metadata:                            < 0.1 %      15.0k

Cache misses by data type:
        Demand data:                                   65.9 %       1.1M
        Prefetch data:                                 31.2 %     534.2k
        Demand metadata:                                1.2 %      20.8k
        Prefetch metadata:                              1.7 %      28.6k

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.