debugfs: Fix memleak in debugfs_change_name().

syzbot reported memleak in debugfs_change_name(). [0]

When lookup_noperm_unlocked() fails, new_name is leaked.

Let's fix it by reusing kfree_const() at the end of
debugfs_change_name().

[0]:
BUG: memory leak
unreferenced object 0xffff8881110bb308 (size 8):
comm "syz.0.17", pid 6090, jiffies 4294942958
hex dump (first 8 bytes):
2e 00 00 00 00 00 00 00 ........
backtrace (crc ecfc7064):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4953 [inline]
slab_alloc_node mm/slub.c:5258 [inline]
__do_kmalloc_node mm/slub.c:5651 [inline]
__kmalloc_node_track_caller_noprof+0x3b2/0x670 mm/slub.c:5759
__kmemdup_nul mm/util.c:64 [inline]
kstrdup+0x3c/0x80 mm/util.c:84
kstrdup_const+0x63/0x80 mm/util.c:104
kvasprintf_const+0xca/0x110 lib/kasprintf.c:48
debugfs_change_name+0xf6/0x5d0 fs/debugfs/inode.c:854
cfg80211_dev_rename+0xd8/0x110 net/wireless/core.c:149
nl80211_set_wiphy+0x102/0x1770 net/wireless/nl80211.c:3844
genl_family_rcv_msg_doit+0x11e/0x190 net/netlink/genetlink.c:1115
genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
genl_rcv_msg+0x2fd/0x440 net/netlink/genetlink.c:1210
netlink_rcv_skb+0x93/0x1d0 net/netlink/af_netlink.c:2550
genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x3a3/0x4f0 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x335/0x6b0 net/netlink/af_netlink.c:1894
sock_sendmsg_nosec net/socket.c:718 [inline]
__sock_sendmsg net/socket.c:733 [inline]
____sys_sendmsg+0x562/0x5a0 net/socket.c:2608
___sys_sendmsg+0xc8/0x130 net/socket.c:2662
__sys_sendmsg+0xc7/0x140 net/socket.c:2694

Fixes: 833d2b3a072f7 ("Add start_renaming_two_dentries()")
Reported-by: [e-mail]
Closes: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
[ Fix minor typo in commit message. - Danilo ]
Signed-off-by: Danilo Krummrich <[e-mail]>

fs/debugfs/inode.c

···
 	rd.new_parent = rd.old_parent;
 	rd.flags = RENAME_NOREPLACE;
 	target = lookup_noperm_unlocked(&QSTR(new_name), rd.new_parent);
-	if (IS_ERR(target))
-		return PTR_ERR(target);
+	if (IS_ERR(target)) {
+		error = PTR_ERR(target);
+		goto out_free;
+	}
 
 	error = start_renaming_two_dentries(&rd, dentry, target);
 	if (error) {
···
 out:
 	dput(rd.old_parent);
 	dput(target);
+out_free:
 	kfree_const(new_name);
 	return error;
 }