frd: a flat ramdisk for the Linux kernel.

The kernel's built-in ramdisk driver (brd.c) nicely doesn't allocate memory
for data that hasn't been written, and keeps a radix tree mapping sectors to
pages that have been written.  While this is good for saving memory, it has
a performance cost, since radix-tree lookups and page allocation are in the
hot paths of reading and writing data.

This driver (frd) is a simpler, dumber, and faster ramdisk that simply
allocates memory for its entire capacity during initialization.