libminixfs, mfs, ext2: may re-evaluate cache size

libminixfs may now be informed of changes to the block usage on the
filesystem. if the net change becomes big enough, libminixfs may
resize the cache based on the new usage.

	. update the 2 FSes to provide this information to libminixfs

Change-Id: I158815a11da801fd5572a8de89c9e6c039b82650
ext2: annotate cache blocks with inode metadata

	. allows mmap() to work on it

Change-Id: I33af8b86ebb408d971478d00ed2caacf15afc7a5