kernel: add do_getpgid() helper; remove internal call to sys_getpgid()

Using the do_getpgid() helper removes an in-kernel call to the
sys_getpgid() syscall.

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

@@ -1027,7 +1027,7 @@ out:
 	return err;
 }
 
-SYSCALL_DEFINE1(getpgid, pid_t, pid)
+static int do_getpgid(pid_t pid)
 {
 	struct task_struct *p;
 	struct pid *grp;
@@ -1055,11 +1055,16 @@ out:
 	return retval;
 }
 
+SYSCALL_DEFINE1(getpgid, pid_t, pid)
+{
+	return do_getpgid(pid);
+}
+
 #ifdef __ARCH_WANT_SYS_GETPGRP
 
 SYSCALL_DEFINE0(getpgrp)
 {
-	return sys_getpgid(0);
+	return do_getpgid(0);
 }
 
 #endif