smp: 64 processors, and say how many there are

NCPU was 16, under what a desktop has today. 64 costs 48 more TSS
I/O bitmaps and scheduler stacks, about 800 KB of BSS. Past 254 the
APIC would need x2APIC mode, which is a separate change.

With more than 16 there were two other ceilings. sched_getaffinity
wrote "CPU 0" and returned 0, and musl clears every byte past the
count returned -- so the mask came back empty and nproc, make -j and
sysconf(_SC_NPROCESSORS_ONLN) all said 1, on any machine. It returns
the mask of processors that started, and its size, now. And /proc
rendered every file into 2 KB, which /proc/cpuinfo ran out of at the
eighteenth processor without a word; /proc/pci, /proc/interrupts and
/proc/mounts were each one bigger machine from the same cut. There is
one 64 KB buffer for it now behind a sleeplock, since there is no
allocator for contiguous pages, and the generators that write raw
are given the real capacity instead of a constant.

Checked under QEMU with -smp 24: nproc 24, /proc/cpuinfo whole.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/file.h

@@ -178,6 +178,11 @@ struct inode {
 // Graft it over `target`; 0 or a negative errno. In sysfs.c.
 int sysfs_mount(const char *target);
 
+// The buffer /proc renders a file into, and how big it is; one at a time, so
+// every get is followed by a put. In kernel/dev/procfs.c.
+char *proc_genbuf_get(int *cap);
+void  proc_genbuf_put(void);
+
 // Mounted FAT volumes, FAT_DEV onward. See include/fat.h.
 
 // Linux's limits, for every filesystem behind the VFS. A path component can be