Two new ways to read a file quickly

Best way to read /proc is to read PAGE_SIZE minimum at once, and interpret short read as EOF for small files like /proc/uptime or /proc/*/statm which are bounded in size. Bigger reads should be (PAGE_SIZE << n) for unbounded files (/proc/*/maps):

m->buf = seq_buf_alloc(m->size <<= 1);

Most of sysfs is 4KB tops but arbitrary sized for binary attributes.