vm: Add missing WITNESS warnings for M_WAITOK allocation

vm_map_clip_{end,start} and lookup_clip_start allocate memory M_WAITOK
for !system_map vm_maps. Add WITNESS warning annotation for !system_map
callers who may be holding non-sleepable locks.

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D25283