tmpfs: clear S_ISGID when setting posix ACLs

commit 497de07 upstream.

This change was missed the tmpfs modification in In CVE-2016-7097
commit 0739310 ("posix_acl: Clear SGID bit when setting
file permissions")
It can test by xfstest generic/375, which failed to clear
setgid bit in the following test case on tmpfs:

  touch $testfile
  chown 100:100 $testfile
  chmod 2755 $testfile
  _runas -u 100 -g 101 -- setfacl -m u::rwx,g::rwx,o::rwx $testfile

Signed-off-by: Gu Zheng <guzheng1@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

fs/generic_acl.c

@@ -82,19 +82,21 @@ generic_acl_set(struct dentry *dentry, const char *name, const void *value,
 			return PTR_ERR(acl);
 	}
 	if (acl) {
+		struct posix_acl *old_acl;
+
 		error = posix_acl_valid(acl);
 		if (error)
 			goto failed;
 		switch (type) {
 		case ACL_TYPE_ACCESS:
-			error = posix_acl_equiv_mode(acl, &inode->i_mode);
+			old_acl = acl;
+			error = posix_acl_update_mode(inode, &inode->i_mode,
+						      &acl);
 			if (error < 0)
 				goto failed;
+			if (!acl)
+				posix_acl_release(old_acl);
 			inode->i_ctime = CURRENT_TIME;
-			if (error == 0) {
-				posix_acl_release(acl);
-				acl = NULL;
-			}
 			break;
 		case ACL_TYPE_DEFAULT:
 			if (!S_ISDIR(inode->i_mode)) {