security: Add LSM hooks to set*gid syscalls

The SafeSetID LSM uses the security_task_fix_setuid hook to filter
set*uid() syscalls according to its configured security policy. In
preparation for adding analagous support in the LSM for set*gid()
syscalls, we add the requisite hook here. Tested by putting print
statements in the security_task_fix_setgid hook and seeing them get hit
during kernel boot.

Signed-off-by: Thomas Cedeno <thomascedeno@google.com>
Signed-off-by: Micah Morton <mortonm@chromium.org>
diff --git a/security/security.c b/security/security.c
index 51de970..5d4ad6f3 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1685,6 +1685,12 @@ int security_task_fix_setuid(struct cred *new, const struct cred *old,
 	return call_int_hook(task_fix_setuid, 0, new, old, flags);
 }
 
+int security_task_fix_setgid(struct cred *new, const struct cred *old,
+				 int flags)
+{
+	return call_int_hook(task_fix_setgid, 0, new, old, flags);
+}
+
 int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
 	return call_int_hook(task_setpgid, 0, p, pgid);