attach_pid() with struct pid parameter

attach_pid() currently takes a pid_t and then uses find_pid() to find the
corresponding struct pid.  Sometimes we already have the struct pid.  We can
then skip find_pid() if attach_pid() were to take a struct pid parameter.

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Cc: <containers@lists.osdl.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/sys.c b/kernel/sys.c
index df4c3a8..872271c 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1488,7 +1488,7 @@
 	if (process_group(p) != pgid) {
 		detach_pid(p, PIDTYPE_PGID);
 		p->signal->pgrp = pgid;
-		attach_pid(p, PIDTYPE_PGID, pgid);
+		attach_pid(p, PIDTYPE_PGID, find_pid(pgid));
 	}
 
 	err = 0;