metadata parser: add a check for invalid values

[royger]

Summary

Add a check to preload_search_info to make sure mod is correctly set. Most
of the callers of preload_search_info don't check that the mod parameter is
valid, which can cause page faults.

Sponsored by: Citrix Systems R&D

[kib]

I would argue that the commit message should talk not about the 'correct' kmdp, but instead of kmdp presence.

There is at least one place which is now redundandly check for kmdp != NULL, look at the hammer_time():1624.

[royger]

Thanks, I've updated the commit message and removed some unnecessary kmdp !=
NULL checks before calling preload_search_info and MD_FETCH.

Currently running a tinderbox, will commit after tinderbox finishes unless
there are other comments.