cachefiles: Set the max subreq size for cache writes to MAX_RW_COUNT

In the Linux kernel, the following vulnerability has been resolved:

cachefiles: Set the max subreq size for cache writes to MAX_RW_COUNT

Set the maximum size of a subrequest that writes to cachefiles to be

MAX_RW_COUNT so that we don't overrun the maximum write we can make to the

backing filesystem.