kernel: add ksys_setsid() helper; remove in-kernel call to sys_setsid()

Using this helper allows us to avoid the in-kernel call to the
sys_setsid() syscall. The ksys_ prefix denotes that this function
is meant as a drop-in replacement for the syscall. In particular, it
uses the same calling convention as sys_setsid().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

kernel/sys.c

@@ -1150,7 +1150,7 @@ static void set_special_pids(struct pid *pid)
 		change_pid(curr, PIDTYPE_PGID, pid);
 }
 
-SYSCALL_DEFINE0(setsid)
+int ksys_setsid(void)
 {
 	struct task_struct *group_leader = current->group_leader;
 	struct pid *sid = task_pid(group_leader);
@@ -1183,6 +1183,11 @@ out:
 	return err;
 }
 
+SYSCALL_DEFINE0(setsid)
+{
+	return ksys_setsid();
+}
+
 DECLARE_RWSEM(uts_sem);
 
 #ifdef COMPAT_UTS_MACHINE