- When cache memory can't be allocated size is recaclulated using 3/4 of
  the requested memory. This number is rounded up to the nearest 
  min_cache step. 
  However with the previous implementation the new cache size might
  become bigger than requested because of this rounding and thus we get
  an infinit loop.
- This patch fixes this problem by ensuring that the new cache size
  always will be smaller on the second and subsequent iterations until
  we reach min_cache.


mysys/mf_iocache.c:
  - Added mask to cachesize to ensure that algorithm always produce a 
    smaller cache size than current, until we reach 'min_cache' size.