We really want f_frsize and f_bavail.

It's confusing, but f_bsize is not the value you're looking for; the
real block size is f_frsize.  Fix all those bugs.

Also, the vast majority of clients are interested in the usable
disk space, not including reserved space.

Test: builds, boots
Bug: 36840579
Change-Id: Ib1470389afd49c14cab62282ec1e978ebb2c4791