VACB lookup and locking is incredibly slow

Testbots are timing out after executing mshtml tests activex, dom, events (and possibly others). This is because the tests write lots of files, and on process exit these get flushed.

In order to flush these, fastfat has to read the FAT numerous times, and each read causes a call to CcMapData. The bottleneck is the lookup and locking of the VACBs in CcRosLookupCacheSegment.
VACBs should be organized in a (sparse multi-level) array instead of a list, and should not require a lock at all.