bdmfs_fatfs is slow

[rickgaiser]

bdmfs_fatfs seems to do many 1 sector (512 byte) reads. These reads are slow, making the new fatfs driver a lot slower in many cases than the old vfat driver.

To fix this issue, the old bdmfs_vfat driver used a sector cache:
https://github.com/ps2dev/ps2sdk/blob/master/iop/fs/bdmfs_vfat/src/scache.c

As a solution for all filesystems, I think it's best to add the cache into BDM itself. So the caching code is shared between all block devices, and all file systems.