fork: adjust sysctl_max_threads definition to match prototype

Commit 32927393 ("sysctl: pass kernel pointers to ->proc_handler")
changed ctl_table.proc_handler to take a kernel pointer.  Adjust the
definition of sysctl_max_threads to match its prototype in
linux/sysctl.h which fixes the following sparse error/warning:

  kernel/fork.c:3050:47: warning: incorrect type in argument 3 (different address spaces)
  kernel/fork.c:3050:47:    expected void *
  kernel/fork.c:3050:47:    got void [noderef] __user *buffer
  kernel/fork.c:3036:5: error: symbol 'sysctl_max_threads' redeclared with different type (incompatible argument 3 (different address spaces)):
  kernel/fork.c:3036:5:    int extern [addressable] [signed] [toplevel] sysctl_max_threads( ... )
  kernel/fork.c: note: in included file (through include/linux/key.h, include/linux/cred.h, include/linux/sched/signal.h, include/linux/sched/cputime.h):
  include/linux/sysctl.h:242:5: note: previously declared as:
  include/linux/sysctl.h:242:5:    int ex...