kernel/utsname_sysctl.c: Fix hostname polling

Commit bfca3dd3 ("kernel/utsname_sysctl.c: print kernel arch") added
a new entry to the uts_kern_table[] array, but didn't update the
UTS_PROC_xyz enumerators of older entries, breaking anything that used
them.

Which is admittedly not many cases: it's really just the two uses of
uts_proc_notify() in kernel/sys.c.  But apparently journald-systemd
actually uses this to detect hostname changes.
Reported-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Fixes: bfca3dd3 ("kernel/utsname_sysctl.c: print kernel arch")
Link: https://lore.kernel.org/lkml/0c2b92a6-0f25-9538-178f-eee3b06da23f@secunet.com/
Link: https://linux-regtracking.leemhuis.info/regzbot/regression/0c2b92a6-0f25-9538-178f-eee3b06da23f@secunet.com/
Cc: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/utsname.h

@@ -10,6 +10,7 @@
 #include <uapi/linux/utsname.h>
 
 enum uts_proc {
+	UTS_PROC_ARCH,
 	UTS_PROC_OSTYPE,
 	UTS_PROC_OSRELEASE,
 	UTS_PROC_VERSION,

kernel/utsname_sysctl.c

@@ -74,6 +74,7 @@ static int proc_do_uts_string(struct ctl_table *table, int write,
 static DEFINE_CTL_TABLE_POLL(hostname_poll);
 static DEFINE_CTL_TABLE_POLL(domainname_poll);
 
+// Note: update 'enum uts_proc' to match any changes to this table
 static struct ctl_table uts_kern_table[] = {
 	{
 		.procname	= "arch",