preload_search_info: make sure mod is set

Add a check to preload_search_info to make sure mod is set. Most of the
callers of preload_search_info don't check that the mod parameter is
set, which can cause page faults. While at it, remove some now unnecessary
checks before calling preload_search_info.

Sponsored by: Citrix Systems R&D
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D3440