We need to be able to store the preferred folio order associated with a
readahead request in the struct file_ra_state so that we can more
accurately increase the order across subsequent readahead requests. But
struct file_ra_state is per-struct file, so we don't really want to
increase it's size.
mmap_miss is currently 32 bits but it is only counted up to 10 *
MMAP_LOTSAMISS, which is currently defined as 1000. So 16 bits should be
plenty. Redefine it to unsigned short, making room for order as unsigned
short in follow up commit.